diff --git a/gems/aws-sdk-s3/CHANGELOG.md b/gems/aws-sdk-s3/CHANGELOG.md
index 7fd43ebaea8..14a390ae6c8 100644
--- a/gems/aws-sdk-s3/CHANGELOG.md
+++ b/gems/aws-sdk-s3/CHANGELOG.md
@@ -1,6 +1,10 @@
 Unreleased Changes
 ------------------
 
+* Feature - Add lightweight thread pool executor for multipart `download_file`, `upload_file` and `upload_stream`.
+
+* Feature - Add custom executor support for `Aws::S3::TransferManager`.
+
 1.199.1 (2025-09-25)
 ------------------
 
diff --git a/gems/aws-sdk-s3/lib/aws-sdk-s3/customizations.rb b/gems/aws-sdk-s3/lib/aws-sdk-s3/customizations.rb
index c0dba64b79c..1ffb0c06892 100644
--- a/gems/aws-sdk-s3/lib/aws-sdk-s3/customizations.rb
+++ b/gems/aws-sdk-s3/lib/aws-sdk-s3/customizations.rb
@@ -7,6 +7,7 @@ module S3
     autoload :Encryption, 'aws-sdk-s3/encryption'
     autoload :EncryptionV2, 'aws-sdk-s3/encryption_v2'
     autoload :FilePart, 'aws-sdk-s3/file_part'
+    autoload :DefaultExecutor, 'aws-sdk-s3/default_executor'
     autoload :FileUploader, 'aws-sdk-s3/file_uploader'
     autoload :FileDownloader, 'aws-sdk-s3/file_downloader'
     autoload :LegacySigner, 'aws-sdk-s3/legacy_signer'
diff --git a/gems/aws-sdk-s3/lib/aws-sdk-s3/customizations/object.rb b/gems/aws-sdk-s3/lib/aws-sdk-s3/customizations/object.rb
index 0a9a9b9d3ca..338eedb87cf 100644
--- a/gems/aws-sdk-s3/lib/aws-sdk-s3/customizations/object.rb
+++ b/gems/aws-sdk-s3/lib/aws-sdk-s3/customizations/object.rb
@@ -358,8 +358,8 @@ def public_url(options = {})
       #   {Client#complete_multipart_upload},
       #   and {Client#upload_part} can be provided.
       #
-      # @option options [Integer] :thread_count (10) The number of parallel
-      #   multipart uploads
+      # @option options [Integer] :thread_count (10) The number of parallel multipart uploads.
+      #   An additional thread is used internally for task coordination.
       #
       # @option options [Boolean] :tempfile (false) Normally read data is stored
       #   in memory when building the parts in order to complete the underlying
@@ -383,19 +383,18 @@ def public_url(options = {})
       # @see Client#complete_multipart_upload
       # @see Client#upload_part
       def upload_stream(options = {}, &block)
-        uploading_options = options.dup
+        upload_opts = options.merge(bucket: bucket_name, key: key)
+        executor = DefaultExecutor.new(max_threads: upload_opts.delete(:thread_count))
         uploader = MultipartStreamUploader.new(
           client: client,
-          thread_count: uploading_options.delete(:thread_count),
-          tempfile: uploading_options.delete(:tempfile),
-          part_size: uploading_options.delete(:part_size)
+          executor: executor,
+          tempfile: upload_opts.delete(:tempfile),
+          part_size: upload_opts.delete(:part_size)
         )
         Aws::Plugins::UserAgent.metric('RESOURCE_MODEL') do
-          uploader.upload(
-            uploading_options.merge(bucket: bucket_name, key: key),
-            &block
-          )
+          uploader.upload(upload_opts, &block)
         end
+        executor.shutdown
         true
       end
       deprecated(:upload_stream, use: 'Aws::S3::TransferManager#upload_stream', version: 'next major version')
@@ -458,12 +457,18 @@ def upload_stream(options = {}, &block)
       # @see Client#complete_multipart_upload
       # @see Client#upload_part
       def upload_file(source, options = {})
-        uploading_options = options.dup
-        uploader = FileUploader.new(multipart_threshold: uploading_options.delete(:multipart_threshold), client: client)
+        upload_opts = options.merge(bucket: bucket_name, key: key)
+        executor = DefaultExecutor.new(max_threads: upload_opts.delete(:thread_count))
+        uploader = FileUploader.new(
+          client: client,
+          executor: executor,
+          multipart_threshold: upload_opts.delete(:multipart_threshold)
+        )
         response = Aws::Plugins::UserAgent.metric('RESOURCE_MODEL') do
-          uploader.upload(source, uploading_options.merge(bucket: bucket_name, key: key))
+          uploader.upload(source, upload_opts)
         end
         yield response if block_given?
+        executor.shutdown
         true
       end
       deprecated(:upload_file, use: 'Aws::S3::TransferManager#upload_file', version: 'next major version')
@@ -512,10 +517,6 @@ def upload_file(source, options = {})
       #
       # @option options [Integer] :thread_count (10) Customize threads used in the multipart download.
       #
-      # @option options [String] :version_id The object version id used to retrieve the object.
-      #
-      #     @see https://docs.aws.amazon.com/AmazonS3/latest/dev/ObjectVersioning.html ObjectVersioning
-      #
       # @option options [String] :checksum_mode ("ENABLED")
       #   When `"ENABLED"` and the object has a stored checksum, it will be used to validate the download and will
       #   raise an `Aws::Errors::ChecksumError` if checksum validation fails. You may provide a `on_checksum_validated`
@@ -539,10 +540,13 @@ def upload_file(source, options = {})
       # @see Client#get_object
       # @see Client#head_object
       def download_file(destination, options = {})
-        downloader = FileDownloader.new(client: client)
+        download_opts = options.merge(bucket: bucket_name, key: key)
+        executor = DefaultExecutor.new(max_threads: download_opts.delete([:thread_count]))
+        downloader = FileDownloader.new(client: client, executor: executor)
         Aws::Plugins::UserAgent.metric('RESOURCE_MODEL') do
-          downloader.download(destination, options.merge(bucket: bucket_name, key: key))
+          downloader.download(destination, download_opts)
         end
+        executor.shutdown
         true
       end
       deprecated(:download_file, use: 'Aws::S3::TransferManager#download_file', version: 'next major version')
diff --git a/gems/aws-sdk-s3/lib/aws-sdk-s3/default_executor.rb b/gems/aws-sdk-s3/lib/aws-sdk-s3/default_executor.rb
new file mode 100644
index 00000000000..13a719f4397
--- /dev/null
+++ b/gems/aws-sdk-s3/lib/aws-sdk-s3/default_executor.rb
@@ -0,0 +1,103 @@
+# frozen_string_literal: true
+
+module Aws
+  module S3
+    # @api private
+    class DefaultExecutor
+      DEFAULT_MAX_THREADS = 10
+      RUNNING = :running
+      SHUTTING_DOWN = :shutting_down
+      SHUTDOWN = :shutdown
+
+      def initialize(options = {})
+        @max_threads = options[:max_threads] || DEFAULT_MAX_THREADS
+        @state = RUNNING
+        @queue = Queue.new
+        @pool = []
+        @mutex = Mutex.new
+      end
+
+      # Submits a task for execution.
+      # @param [Object] args Variable number of arguments to pass to the block
+      # @param [Proc] block The block to be executed
+      # @return [Boolean] Returns true if the task was submitted successfully
+      def post(*args, &block)
+        @mutex.synchronize do
+          raise 'Executor has been shutdown and is no longer accepting tasks' unless @state == RUNNING
+
+          @queue << [args, block]
+          ensure_worker_available
+        end
+        true
+      end
+
+      # Immediately terminates all worker threads and clears pending tasks.
+      # This is a forceful shutdown that doesn't wait for running tasks to complete.
+      #
+      # @return [Boolean] true when termination is complete
+      def kill
+        @mutex.synchronize do
+          @state = SHUTDOWN
+          @pool.each(&:kill)
+          @pool.clear
+          @queue.clear
+        end
+        true
+      end
+
+      # Gracefully shuts down the executor, optionally with a timeout.
+      # Stops accepting new tasks and waits for running tasks to complete.
+      #
+      # @param timeout [Numeric, nil] Maximum time in seconds to wait for shutdown.
+      #   If nil, waits indefinitely. If timeout expires, remaining threads are killed.
+      # @return [Boolean] true when shutdown is complete
+      def shutdown(timeout = nil)
+        @mutex.synchronize do
+          return true if @state == SHUTDOWN
+
+          @state = SHUTTING_DOWN
+          @pool.size.times { @queue << :shutdown }
+        end
+
+        if timeout
+          deadline = Time.now + timeout
+          @pool.each do |thread|
+            remaining = deadline - Time.now
+            break if remaining <= 0
+
+            thread.join([remaining, 0].max)
+          end
+          @pool.select(&:alive?).each(&:kill)
+        else
+          @pool.each(&:join)
+        end
+
+        @mutex.synchronize do
+          @pool.clear
+          @state = SHUTDOWN
+        end
+        true
+      end
+
+      private
+
+      def ensure_worker_available
+        return unless @state == RUNNING
+
+        @pool.select!(&:alive?)
+        @pool << spawn_worker if @pool.size < @max_threads
+      end
+
+      def spawn_worker
+        Thread.new do
+          while (job = @queue.shift)
+            break if job == :shutdown
+
+            args, block = job
+            block.call(*args)
+          end
+        end
+      end
+    end
+  end
+end
diff --git a/gems/aws-sdk-s3/lib/aws-sdk-s3/file_downloader.rb b/gems/aws-sdk-s3/lib/aws-sdk-s3/file_downloader.rb
index 517a06dabea..7e8f545cb01 100644
--- a/gems/aws-sdk-s3/lib/aws-sdk-s3/file_downloader.rb
+++ b/gems/aws-sdk-s3/lib/aws-sdk-s3/file_downloader.rb
@@ -8,184 +8,245 @@ module Aws
   module S3
     # @api private
     class FileDownloader
-
       MIN_CHUNK_SIZE = 5 * 1024 * 1024
       MAX_PARTS = 10_000
+      HEAD_OPTIONS = Set.new(Client.api.operation(:head_object).input.shape.member_names)
+      GET_OPTIONS = Set.new(Client.api.operation(:get_object).input.shape.member_names)
 
       def initialize(options = {})
         @client = options[:client] || Client.new
+        @executor = options[:executor]
       end
 
       # @return [Client]
       attr_reader :client
 
       def download(destination, options = {})
-        valid_types = [String, Pathname, File, Tempfile]
-        unless valid_types.include?(destination.class)
-          raise ArgumentError, "Invalid destination, expected #{valid_types.join(', ')} but got: #{destination.class}"
-        end
-
-        @destination = destination
-        @mode = options.delete(:mode) || 'auto'
-        @thread_count = options.delete(:thread_count) || 10
-        @chunk_size = options.delete(:chunk_size)
-        @on_checksum_validated = options.delete(:on_checksum_validated)
-        @progress_callback = options.delete(:progress_callback)
-        @params = options
-        validate!
+        validate_destination!(destination)
+        opts = build_download_opts(destination, options)
+        validate_opts!(opts)
 
         Aws::Plugins::UserAgent.metric('S3_TRANSFER') do
-          case @mode
-          when 'auto' then multipart_download
-          when 'single_request' then single_request
-          when 'get_range'
-            raise ArgumentError, 'In get_range mode, :chunk_size must be provided' unless @chunk_size
-
-            resp = @client.head_object(@params)
-            multithreaded_get_by_ranges(resp.content_length, resp.etag)
-          else
-            raise ArgumentError, "Invalid mode #{@mode} provided, :mode should be single_request, get_range or auto"
+          case opts[:mode]
+          when 'auto' then multipart_download(opts)
+          when 'single_request' then single_request(opts)
+          when 'get_range' then range_request(opts)
           end
         end
-        File.rename(@temp_path, @destination) if @temp_path
+        File.rename(opts[:temp_path], destination) if opts[:temp_path]
       ensure
-        File.delete(@temp_path) if @temp_path && File.exist?(@temp_path)
+        cleanup_temp_file(opts)
       end
 
       private
 
-      def validate!
-        return unless @on_checksum_validated && !@on_checksum_validated.respond_to?(:call)
+      def build_download_opts(destination, opts)
+        {
+          destination: destination,
+          mode: opts.delete(:mode) || 'auto',
+          chunk_size: opts.delete(:chunk_size),
+          on_checksum_validated: opts.delete(:on_checksum_validated),
+          progress_callback: opts.delete(:progress_callback),
+          params: opts,
+          temp_path: nil
+        }
+      end
+
+      def cleanup_temp_file(opts)
+        return unless opts
+
+        temp_file = opts[:temp_path]
+        File.delete(temp_file) if temp_file && File.exist?(temp_file)
+      end
+
+      def download_with_executor(part_list, total_size, opts)
+        download_attempts = 0
+        completion_queue = Queue.new
+        abort_download = false
+        error = nil
+        progress = MultipartProgress.new(part_list, total_size, opts[:progress_callback])
+
+        while (part = part_list.shift)
+          break if abort_download
+
+          download_attempts += 1
+          @executor.post(part) do |p|
+            update_progress(progress, p)
+            resp = @client.get_object(p.params)
+            range = extract_range(resp.content_range)
+            validate_range(range, p.params[:range]) if p.params[:range]
+            write(resp.body, range, opts)
+
+            execute_checksum_callback(resp, opts)
+          rescue StandardError => e
+            abort_download = true
+            error = e
+          ensure
+            completion_queue << :done
+          end
+        end
+
+        download_attempts.times { completion_queue.pop }
+        raise error unless error.nil?
+      end
+
+      def get_opts(opts)
+        GET_OPTIONS.each_with_object({}) { |k, h| h[k] = opts[k] if opts.key?(k) }
+      end
+
+      def head_opts(opts)
+        HEAD_OPTIONS.each_with_object({}) { |k, h| h[k] = opts[k] if opts.key?(k) }
+      end
+
+      def compute_chunk(chunk_size, file_size)
+        raise ArgumentError, ":chunk_size shouldn't exceed total file size." if chunk_size && chunk_size > file_size
 
-        raise ArgumentError, ':on_checksum_validated must be callable'
+        chunk_size || [(file_size.to_f / MAX_PARTS).ceil, MIN_CHUNK_SIZE].max.to_i
       end
 
-      def multipart_download
-        resp = @client.head_object(@params.merge(part_number: 1))
+      def compute_mode(file_size, total_parts, etag, opts)
+        chunk_size = compute_chunk(opts[:chunk_size], file_size)
+        part_size = (file_size.to_f / total_parts).ceil
+
+        resolve_temp_path(opts)
+        if chunk_size < part_size
+          multithreaded_get_by_ranges(file_size, etag, opts)
+        else
+          multithreaded_get_by_parts(total_parts, file_size, etag, opts)
+        end
+      end
+
+      def extract_range(value)
+        value.match(%r{bytes (?<range>\d+-\d+)/\d+})[:range]
+      end
+
+      def multipart_download(opts)
+        resp = @client.head_object(head_opts(opts[:params].merge(part_number: 1)))
         count = resp.parts_count
 
         if count.nil? || count <= 1
           if resp.content_length <= MIN_CHUNK_SIZE
-            single_request
+            single_request(opts)
           else
-            multithreaded_get_by_ranges(resp.content_length, resp.etag)
+            resolve_temp_path(opts)
+            multithreaded_get_by_ranges(resp.content_length, resp.etag, opts)
           end
         else
           # covers cases when given object is not uploaded via UploadPart API
-          resp = @client.head_object(@params) # partNumber is an option
+          resp = @client.head_object(head_opts(opts[:params])) # partNumber is an option
           if resp.content_length <= MIN_CHUNK_SIZE
-            single_request
+            single_request(opts)
           else
-            compute_mode(resp.content_length, count, resp.etag)
+            compute_mode(resp.content_length, count, resp.etag, opts)
           end
         end
       end
 
-      def compute_mode(file_size, count, etag)
-        chunk_size = compute_chunk(file_size)
-        part_size = (file_size.to_f / count).ceil
-        if chunk_size < part_size
-          multithreaded_get_by_ranges(file_size, etag)
-        else
-          multithreaded_get_by_parts(count, file_size, etag)
+      def multithreaded_get_by_parts(total_parts, file_size, etag, opts)
+        parts = (1..total_parts).map do |part|
+          params = get_opts(opts[:params].merge(part_number: part, if_match: etag))
+          Part.new(part_number: part, params: params)
         end
+        download_with_executor(PartList.new(parts), file_size, opts)
       end
 
-      def compute_chunk(file_size)
-        raise ArgumentError, ":chunk_size shouldn't exceed total file size." if @chunk_size && @chunk_size > file_size
-
-        @chunk_size || [(file_size.to_f / MAX_PARTS).ceil, MIN_CHUNK_SIZE].max.to_i
-      end
-
-      def multithreaded_get_by_ranges(file_size, etag)
+      def multithreaded_get_by_ranges(file_size, etag, opts)
         offset = 0
-        default_chunk_size = compute_chunk(file_size)
+        default_chunk_size = compute_chunk(opts[:chunk_size], file_size)
         chunks = []
         part_number = 1 # parts start at 1
         while offset < file_size
           progress = offset + default_chunk_size
           progress = file_size if progress > file_size
-          params = @params.merge(range: "bytes=#{offset}-#{progress - 1}", if_match: etag)
+          params = get_opts(opts[:params].merge(range: "bytes=#{offset}-#{progress - 1}", if_match: etag))
           chunks << Part.new(part_number: part_number, size: (progress - offset), params: params)
           part_number += 1
           offset = progress
         end
-        download_in_threads(PartList.new(chunks), file_size)
-      end
-
-      def multithreaded_get_by_parts(n_parts, total_size, etag)
-        parts = (1..n_parts).map do |part|
-          Part.new(part_number: part, params: @params.merge(part_number: part, if_match: etag))
-        end
-        download_in_threads(PartList.new(parts), total_size)
-      end
-
-      def download_in_threads(pending, total_size)
-        threads = []
-        progress = MultipartProgress.new(pending, total_size, @progress_callback) if @progress_callback
-        unless [File, Tempfile].include?(@destination.class)
-          @temp_path = "#{@destination}.s3tmp.#{SecureRandom.alphanumeric(8)}"
-        end
-        @thread_count.times do
-          thread = Thread.new do
-            begin
-              while (part = pending.shift)
-                if progress
-                  part.params[:on_chunk_received] =
-                    proc do |_chunk, bytes, total|
-                      progress.call(part.part_number, bytes, total)
-                    end
-                end
-                resp = @client.get_object(part.params)
-                range = extract_range(resp.content_range)
-                validate_range(range, part.params[:range]) if part.params[:range]
-                write(resp.body, range)
-                if @on_checksum_validated && resp.checksum_validated
-                  @on_checksum_validated.call(resp.checksum_validated, resp)
-                end
-              end
-              nil
-            rescue StandardError => e
-              pending.clear! # keep other threads from downloading other parts
-              raise e
-            end
-          end
-          threads << thread
-        end
-        threads.map(&:value).compact
+        download_with_executor(PartList.new(chunks), file_size, opts)
       end
 
-      def extract_range(value)
-        value.match(%r{bytes (?<range>\d+-\d+)/\d+})[:range]
+      def range_request(opts)
+        resp = @client.head_object(head_opts(opts[:params]))
+        resolve_temp_path(opts)
+        multithreaded_get_by_ranges(resp.content_length, resp.etag, opts)
       end
 
-      def validate_range(actual, expected)
-        return if actual == expected.match(/bytes=(?<range>\d+-\d+)/)[:range]
-
-        raise MultipartDownloadError, "multipart download failed: expected range of #{expected} but got #{actual}"
-      end
+      def resolve_temp_path(opts)
+        return if [File, Tempfile].include?(opts[:destination].class)
 
-      def write(body, range)
-        path = @temp_path || @destination
-        File.write(path, body.read, range.split('-').first.to_i)
+        opts[:temp_path] ||= "#{opts[:destination]}.s3tmp.#{SecureRandom.alphanumeric(8)}"
       end
 
-      def single_request
-        params = @params.merge(response_target: @destination)
-        params[:on_chunk_received] = single_part_progress if @progress_callback
+      def single_request(opts)
+        params = get_opts(opts[:params]).merge(response_target: opts[:destination])
+        params[:on_chunk_received] = single_part_progress(opts) if opts[:progress_callback]
         resp = @client.get_object(params)
-        return resp unless @on_checksum_validated
+        return resp unless opts[:on_checksum_validated]
 
-        @on_checksum_validated.call(resp.checksum_validated, resp) if resp.checksum_validated
+        opts[:on_checksum_validated].call(resp.checksum_validated, resp) if resp.checksum_validated
         resp
       end
 
-      def single_part_progress
+      def single_part_progress(opts)
         proc do |_chunk, bytes_read, total_size|
-          @progress_callback.call([bytes_read], [total_size], total_size)
+          opts[:progress_callback].call([bytes_read], [total_size], total_size)
         end
       end
 
+      def update_progress(progress, part)
+        return unless progress.progress_callback
+
+        part.params[:on_chunk_received] =
+          proc do |_chunk, bytes, total|
+            progress.call(part.part_number, bytes, total)
+          end
+      end
+
+      def execute_checksum_callback(resp, opts)
+        return unless opts[:on_checksum_validated] && resp.checksum_validated
+
+        opts[:on_checksum_validated].call(resp.checksum_validated, resp)
+      end
+
+      def validate_destination!(destination)
+        valid_types = [String, Pathname, File, Tempfile]
+        return if valid_types.include?(destination.class)
+
+        raise ArgumentError, "Invalid destination, expected #{valid_types.join(', ')} but got: #{destination.class}"
+      end
+
+      def validate_opts!(opts)
+        if opts[:on_checksum_validated] && !opts[:on_checksum_validated].respond_to?(:call)
+          raise ArgumentError, ':on_checksum_validated must be callable'
+        end
+
+        valid_modes = %w[auto get_range single_request]
+        unless valid_modes.include?(opts[:mode])
+          msg = "Invalid mode #{opts[:mode]} provided, :mode should be single_request, get_range or auto"
+          raise ArgumentError, msg
+        end
+
+        if opts[:mode] == 'get_range' && opts[:chunk_size].nil?
+          raise ArgumentError, 'In get_range mode, :chunk_size must be provided'
+        end
+
+        if opts[:chunk_size] && opts[:chunk_size] <= 0
+          raise ArgumentError, ':chunk_size must be positive'
+        end
+      end
+
+      def validate_range(actual, expected)
+        return if actual == expected.match(/bytes=(?<range>\d+-\d+)/)[:range]
+
+        raise MultipartDownloadError, "multipart download failed: expected range of #{expected} but got #{actual}"
+      end
+
+      def write(body, range, opts)
+        path = opts[:temp_path] || opts[:destination]
+        File.write(path, body.read, range.split('-').first.to_i)
+      end
+
       # @api private
       class Part < Struct.new(:part_number, :size, :params)
         include Aws::Structure
@@ -225,6 +286,8 @@ def initialize(parts, total_size, progress_callback)
           @progress_callback = progress_callback
         end
 
+        attr_reader :progress_callback
+
         def call(part_number, bytes_received, total)
           # part numbers start at 1
           @bytes_received[part_number - 1] = bytes_received
diff --git a/gems/aws-sdk-s3/lib/aws-sdk-s3/file_uploader.rb b/gems/aws-sdk-s3/lib/aws-sdk-s3/file_uploader.rb
index 587066551ea..62dbb07f8c3 100644
--- a/gems/aws-sdk-s3/lib/aws-sdk-s3/file_uploader.rb
+++ b/gems/aws-sdk-s3/lib/aws-sdk-s3/file_uploader.rb
@@ -13,8 +13,8 @@ class FileUploader
       # @option options [Client] :client
       # @option options [Integer] :multipart_threshold (104857600)
       def initialize(options = {})
-        @options = options
         @client = options[:client] || Client.new
+        @executor = options[:executor]
         @multipart_threshold = options[:multipart_threshold] || DEFAULT_MULTIPART_THRESHOLD
       end
 
@@ -36,11 +36,9 @@ def initialize(options = {})
       # @return [void]
       def upload(source, options = {})
         Aws::Plugins::UserAgent.metric('S3_TRANSFER') do
-          if File.size(source) >= multipart_threshold
-            MultipartFileUploader.new(@options).upload(source, options)
+          if File.size(source) >= @multipart_threshold
+            MultipartFileUploader.new(client: @client, executor: @executor).upload(source, options)
           else
-            # remove multipart parameters not supported by put_object
-            options.delete(:thread_count)
             put_object(source, options)
           end
         end
@@ -48,9 +46,9 @@ def upload(source, options = {})
 
       private
 
-      def open_file(source)
-        if String === source || Pathname === source
-          File.open(source, 'rb') { |file| yield(file) }
+      def open_file(source, &block)
+        if source.is_a?(String) || source.is_a?(Pathname)
+          File.open(source, 'rb', &block)
         else
           yield(source)
         end
diff --git a/gems/aws-sdk-s3/lib/aws-sdk-s3/multipart_file_uploader.rb b/gems/aws-sdk-s3/lib/aws-sdk-s3/multipart_file_uploader.rb
index bcc05f1fc9e..e1f05d6ee16 100644
--- a/gems/aws-sdk-s3/lib/aws-sdk-s3/multipart_file_uploader.rb
+++ b/gems/aws-sdk-s3/lib/aws-sdk-s3/multipart_file_uploader.rb
@@ -7,10 +7,8 @@ module Aws
   module S3
     # @api private
     class MultipartFileUploader
-
       MIN_PART_SIZE = 5 * 1024 * 1024 # 5MB
       MAX_PARTS = 10_000
-      DEFAULT_THREAD_COUNT = 10
       CREATE_OPTIONS = Set.new(Client.api.operation(:create_multipart_upload).input.shape.member_names)
       COMPLETE_OPTIONS = Set.new(Client.api.operation(:complete_multipart_upload).input.shape.member_names)
       UPLOAD_PART_OPTIONS = Set.new(Client.api.operation(:upload_part).input.shape.member_names)
@@ -21,10 +19,9 @@ class MultipartFileUploader
       )
 
       # @option options [Client] :client
-      # @option options [Integer] :thread_count (DEFAULT_THREAD_COUNT)
       def initialize(options = {})
         @client = options[:client] || Client.new
-        @thread_count = options[:thread_count] || DEFAULT_THREAD_COUNT
+        @executor = options[:executor]
       end
 
       # @return [Client]
@@ -38,11 +35,12 @@ def initialize(options = {})
       #   It will be invoked with [bytes_read], [total_sizes]
       # @return [Seahorse::Client::Response] - the CompleteMultipartUploadResponse
       def upload(source, options = {})
-        raise ArgumentError, 'unable to multipart upload files smaller than 5MB' if File.size(source) < MIN_PART_SIZE
+        file_size = File.size(source)
+        raise ArgumentError, 'unable to multipart upload files smaller than 5MB' if file_size < MIN_PART_SIZE
 
         upload_id = initiate_upload(options)
-        parts = upload_parts(upload_id, source, options)
-        complete_upload(upload_id, parts, source, options)
+        parts = upload_parts(upload_id, source, file_size, options)
+        complete_upload(upload_id, parts, file_size, options)
       end
 
       private
@@ -51,22 +49,22 @@ def initiate_upload(options)
         @client.create_multipart_upload(create_opts(options)).upload_id
       end
 
-      def complete_upload(upload_id, parts, source, options)
+      def complete_upload(upload_id, parts, file_size, options)
         @client.complete_multipart_upload(
           **complete_opts(options).merge(
             upload_id: upload_id,
             multipart_upload: { parts: parts },
-            mpu_object_size: File.size(source)
+            mpu_object_size: file_size
           )
         )
       rescue StandardError => e
         abort_upload(upload_id, options, [e])
       end
 
-      def upload_parts(upload_id, source, options)
+      def upload_parts(upload_id, source, file_size, options)
         completed = PartList.new
-        pending = PartList.new(compute_parts(upload_id, source, options))
-        errors = upload_in_threads(pending, completed, options)
+        pending = PartList.new(compute_parts(upload_id, source, file_size, options))
+        errors = upload_with_executor(pending, completed, options)
         if errors.empty?
           completed.to_a.sort_by { |part| part[:part_number] }
         else
@@ -86,17 +84,20 @@ def abort_upload(upload_id, options, errors)
         raise MultipartUploadError.new(msg, errors + [e])
       end
 
-      def compute_parts(upload_id, source, options)
-        size = File.size(source)
-        default_part_size = compute_default_part_size(size)
+      def compute_parts(upload_id, source, file_size, options)
+        default_part_size = compute_default_part_size(file_size)
         offset = 0
         part_number = 1
         parts = []
-        while offset < size
+        while offset < file_size
           parts << upload_part_opts(options).merge(
             upload_id: upload_id,
             part_number: part_number,
-            body: FilePart.new(source: source, offset: offset, size: part_size(size, default_part_size, offset))
+            body: FilePart.new(
+              source: source,
+              offset: offset,
+              size: part_size(file_size, default_part_size, offset)
+            )
           )
           part_number += 1
           offset += default_part_size
@@ -115,17 +116,13 @@ def has_checksum_key?(keys)
       def create_opts(options)
         opts = { checksum_algorithm: Aws::Plugins::ChecksumAlgorithm::DEFAULT_CHECKSUM }
         opts[:checksum_type] = 'FULL_OBJECT' if has_checksum_key?(options.keys)
-        CREATE_OPTIONS.each_with_object(opts) do |key, hash|
-          hash[key] = options[key] if options.key?(key)
-        end
+        CREATE_OPTIONS.each_with_object(opts) { |k, h| h[k] = options[k] if options.key?(k) }
       end
 
       def complete_opts(options)
         opts = {}
         opts[:checksum_type] = 'FULL_OBJECT' if has_checksum_key?(options.keys)
-        COMPLETE_OPTIONS.each_with_object(opts) do |key, hash|
-          hash[key] = options[key] if options.key?(key)
-        end
+        COMPLETE_OPTIONS.each_with_object(opts) { |k, h| h[k] = options[k] if options.key?(k) }
       end
 
       def upload_part_opts(options)
@@ -135,43 +132,40 @@ def upload_part_opts(options)
         end
       end
 
-      def upload_in_threads(pending, completed, options)
-        threads = []
-        if (callback = options[:progress_callback])
-          progress = MultipartProgress.new(pending, callback)
-        end
-        options.fetch(:thread_count, @thread_count).times do
-          thread = Thread.new do
-            begin
-              while (part = pending.shift)
-                if progress
-                  part[:on_chunk_sent] =
-                    proc do |_chunk, bytes, _total|
-                      progress.call(part[:part_number], bytes)
-                    end
-                end
-                resp = @client.upload_part(part)
-                part[:body].close
-                completed_part = { etag: resp.etag, part_number: part[:part_number] }
-                algorithm = resp.context.params[:checksum_algorithm]
-                k = "checksum_#{algorithm.downcase}".to_sym
-                completed_part[k] = resp.send(k)
-                completed.push(completed_part)
-              end
-              nil
-            rescue StandardError => e
-              # keep other threads from uploading other parts
-              pending.clear!
-              e
-            end
+      def upload_with_executor(pending, completed, options)
+        upload_attempts = 0
+        completion_queue = Queue.new
+        abort_upload = false
+        errors = []
+        progress = MultipartProgress.new(pending, options[:progress_callback])
+
+        while (part = pending.shift)
+          break if abort_upload
+
+          upload_attempts += 1
+          @executor.post(part) do |p|
+            update_progress(progress, p)
+            resp = @client.upload_part(p)
+            p[:body].close
+            completed_part = { etag: resp.etag, part_number: p[:part_number] }
+            algorithm = resp.context.params[:checksum_algorithm].downcase
+            k = "checksum_#{algorithm}".to_sym
+            completed_part[k] = resp.send(k)
+            completed.push(completed_part)
+          rescue StandardError => e
+            abort_upload = true
+            errors << e
+          ensure
+            completion_queue << :done
           end
-          threads << thread
         end
-        threads.map(&:value).compact
+
+        upload_attempts.times { completion_queue.pop }
+        errors
       end
 
-      def compute_default_part_size(source_size)
-        [(source_size.to_f / MAX_PARTS).ceil, MIN_PART_SIZE].max.to_i
+      def compute_default_part_size(file_size)
+        [(file_size.to_f / MAX_PARTS).ceil, MIN_PART_SIZE].max.to_i
       end
 
       def part_size(total_size, part_size, offset)
@@ -182,6 +176,15 @@ def part_size(total_size, part_size, offset)
         end
       end
 
+      def update_progress(progress, part)
+        return unless progress.progress_callback
+
+        part[:on_chunk_sent] =
+          proc do |_chunk, bytes, _total|
+            progress.call(part[:part_number], bytes)
+          end
+      end
+
       # @api private
       class PartList
         def initialize(parts = [])
@@ -222,6 +225,8 @@ def initialize(parts, progress_callback)
           @progress_callback = progress_callback
         end
 
+        attr_reader :progress_callback
+
         def call(part_number, bytes_read)
           # part numbers start at 1
           @bytes_sent[part_number - 1] = bytes_read
diff --git a/gems/aws-sdk-s3/lib/aws-sdk-s3/multipart_stream_uploader.rb b/gems/aws-sdk-s3/lib/aws-sdk-s3/multipart_stream_uploader.rb
index 60a298c720b..ae6f75a47a1 100644
--- a/gems/aws-sdk-s3/lib/aws-sdk-s3/multipart_stream_uploader.rb
+++ b/gems/aws-sdk-s3/lib/aws-sdk-s3/multipart_stream_uploader.rb
@@ -11,7 +11,6 @@ module S3
     class MultipartStreamUploader
 
       DEFAULT_PART_SIZE = 5 * 1024 * 1024 # 5MB
-      DEFAULT_THREAD_COUNT = 10
       CREATE_OPTIONS = Set.new(Client.api.operation(:create_multipart_upload).input.shape.member_names)
       UPLOAD_PART_OPTIONS = Set.new(Client.api.operation(:upload_part).input.shape.member_names)
       COMPLETE_UPLOAD_OPTIONS = Set.new(Client.api.operation(:complete_multipart_upload).input.shape.member_names)
@@ -19,9 +18,9 @@ class MultipartStreamUploader
       # @option options [Client] :client
       def initialize(options = {})
         @client = options[:client] || Client.new
+        @executor = options[:executor]
         @tempfile = options[:tempfile]
         @part_size = options[:part_size] || DEFAULT_PART_SIZE
-        @thread_count = options[:thread_count] || DEFAULT_THREAD_COUNT
       end
 
       # @return [Client]
@@ -29,7 +28,6 @@ def initialize(options = {})
 
       # @option options [required,String] :bucket
       # @option options [required,String] :key
-      # @option options [Integer] :thread_count (DEFAULT_THREAD_COUNT)
       # @return [Seahorse::Client::Response] - the CompleteMultipartUploadResponse
       def upload(options = {}, &block)
         Aws::Plugins::UserAgent.metric('S3_TRANSFER') do
@@ -54,28 +52,30 @@ def complete_upload(upload_id, parts, options)
       end
 
       def upload_parts(upload_id, options, &block)
-        completed = Queue.new
-        thread_errors = []
-        errors = begin
+        completed_parts = Queue.new
+        errors = []
+
+        begin
           IO.pipe do |read_pipe, write_pipe|
-            threads = upload_in_threads(
-              read_pipe,
-              completed,
-              upload_part_opts(options).merge(upload_id: upload_id),
-              thread_errors
-            )
-            begin
-              block.call(write_pipe)
-            ensure
-              # Ensure the pipe is closed to avoid https://github.com/jruby/jruby/issues/6111
-              write_pipe.close
+            upload_thread = Thread.new do
+              upload_with_executor(
+                read_pipe,
+                completed_parts,
+                errors,
+                upload_part_opts(options).merge(upload_id: upload_id)
+              )
             end
-            threads.map(&:value).compact
+
+            block.call(write_pipe)
+          ensure
+            # Ensure the pipe is closed to avoid https://github.com/jruby/jruby/issues/6111
+            write_pipe.close
+            upload_thread.join
           end
         rescue StandardError => e
-          thread_errors + [e]
+          errors << e
         end
-        return ordered_parts(completed) if errors.empty?
+        return ordered_parts(completed_parts) if errors.empty?
 
         abort_upload(upload_id, options, errors)
       end
@@ -128,37 +128,34 @@ def read_to_part_body(read_pipe)
         end
       end
 
-      def upload_in_threads(read_pipe, completed, options, thread_errors)
-        mutex = Mutex.new
+      def upload_with_executor(read_pipe, completed, errors, options)
+        completion_queue = Queue.new
+        queued_parts = 0
         part_number = 0
-        options.fetch(:thread_count, @thread_count).times.map do
-          thread = Thread.new do
-            loop do
-              body, thread_part_number = mutex.synchronize do
-                [read_to_part_body(read_pipe), part_number += 1]
-              end
-              break unless body || thread_part_number == 1
-
-              begin
-                part = options.merge(body: body, part_number: thread_part_number)
-                resp = @client.upload_part(part)
-                completed_part = create_completed_part(resp, part)
-                completed.push(completed_part)
-              ensure
-                clear_body(body)
-              end
-            end
-            nil
+        mutex = Mutex.new
+        loop do
+          part_body, current_part_num = mutex.synchronize do
+            [read_to_part_body(read_pipe), part_number += 1]
+          end
+          break unless part_body || current_part_num == 1
+
+          queued_parts += 1
+          @executor.post(part_body, current_part_num, options) do |body, num, opts|
+            part = opts.merge(body: body, part_number: num)
+            resp = @client.upload_part(part)
+            completed_part = create_completed_part(resp, part)
+            completed.push(completed_part)
           rescue StandardError => e
-            # keep other threads from uploading other parts
             mutex.synchronize do
-              thread_errors.push(e)
+              errors.push(e)
               read_pipe.close_read unless read_pipe.closed?
             end
-            e
+          ensure
+            clear_body(body)
+            completion_queue << :done
           end
-          thread
         end
+        queued_parts.times { completion_queue.pop }
       end
 
       def create_completed_part(resp, part)
diff --git a/gems/aws-sdk-s3/lib/aws-sdk-s3/transfer_manager.rb b/gems/aws-sdk-s3/lib/aws-sdk-s3/transfer_manager.rb
index 03bd249d5e2..9035e9699ac 100644
--- a/gems/aws-sdk-s3/lib/aws-sdk-s3/transfer_manager.rb
+++ b/gems/aws-sdk-s3/lib/aws-sdk-s3/transfer_manager.rb
@@ -2,27 +2,74 @@
 
 module Aws
   module S3
-    # A high-level S3 transfer utility that provides enhanced upload and download
-    # capabilities with automatic multipart handling, progress tracking, and
-    # handling of large files. The following features are supported:
+    # A high-level S3 transfer utility that provides enhanced upload and download capabilities with automatic
+    # multipart handling, progress tracking, and handling of large files. The following features are supported:
     #
     # * upload a file with multipart upload
     # * upload a stream with multipart upload
     # * download a S3 object with multipart download
     # * track transfer progress by using progress listener
     #
+    # ## Executor Management
+    # TransferManager uses executors to handle concurrent operations during multipart transfers. You can control
+    # concurrency behavior by providing a custom executor or relying on the default executor management.
+    #
+    # ### Default Behavior
+    # When no `:executor` is provided, TransferManager creates a new DefaultExecutor for each individual
+    # operation (`download_file`, `upload_file`, etc.) and automatically shuts it down when that operation completes.
+    # Each operation gets its own isolated thread pool with the specified `:thread_count` (default 10 threads).
+    #
+    # ### Custom Executor
+    # You can provide your own executor (e.g., `Concurrent::ThreadPoolExecutor`) for fine-grained control over thread
+    # pools and resource management. When using a custom executor, you are responsible for shutting it down
+    # when finished. The executor may be reused across multiple TransferManager operations.
+    #
+    # Custom executors must implement the same interface as DefaultExecutor.
+    #
+    # **Required methods:**
+    #
+    #   * `post(*args, &block)` - Execute a task with given arguments and block
+    #   * `kill` - Immediately terminate all running tasks
+    #
+    # **Optional methods:**
+    #
+    #   * `shutdown(timeout = nil)` - Gracefully shutdown the executor with optional timeout
+    #
+    # @example Using default executor (automatic creation and shutdown)
+    #     tm = TransferManager.new # No executor provided
+    #     # DefaultExecutor created, used, and shutdown automatically
+    #     tm.download_file('/path/to/file', bucket: 'bucket', key: 'key')
+    #
+    # @example Using custom executor (manual shutdown required)
+    #     require 'concurrent-ruby'
+    #
+    #     executor = Concurrent::ThreadPoolExecutor.new(max_threads: 5)
+    #     tm = TransferManager.new(executor: executor)
+    #     tm.download_file('/path/to/file1', bucket: 'bucket', key: 'key1')
+    #     executor.shutdown # You must shutdown custom executors
+    #
     class TransferManager
+
       # @param [Hash] options
       # @option options [S3::Client] :client (S3::Client.new)
       #   The S3 client to use for {TransferManager} operations. If not provided, a new default client
       #   will be created automatically.
+      # @option options [Object] :executor
+      #   The executor to use for multipart operations. Must implement the same interface as {DefaultExecutor}.
+      #   If not provided, a new {DefaultExecutor} will be created automatically for each operation and
+      #   shutdown after completion. When provided a custom executor, it will be reused across operations, and
+      #   you are responsible for shutting it down when finished.
       def initialize(options = {})
-        @client = options.delete(:client) || Client.new
+        @client = options[:client] || Client.new
+        @executor = options[:executor]
       end
 
       # @return [S3::Client]
       attr_reader :client
 
+      # @return [Object]
+      attr_reader :executor
+
       # Downloads a file in S3 to a path on disk.
       #
       #     # small files (< 5MB) are downloaded in a single API call
@@ -74,10 +121,7 @@ def initialize(options = {})
       # @option options [Integer] :chunk_size required in `"get_range"` mode.
       #
       # @option options [Integer] :thread_count (10) Customize threads used in the multipart download.
-      #
-      # @option options [String] :version_id The object version id used to retrieve the object.
-      #
-      #     @see https://docs.aws.amazon.com/AmazonS3/latest/dev/ObjectVersioning.html ObjectVersioning
+      #   Only used when no custom executor is provided (creates {DefaultExecutor} with given thread count).
       #
       # @option options [String] :checksum_mode ("ENABLED")
       #   When `"ENABLED"` and the object has a stored checksum, it will be used to validate the download and will
@@ -102,8 +146,11 @@ def initialize(options = {})
       # @see Client#get_object
       # @see Client#head_object
       def download_file(destination, bucket:, key:, **options)
-        downloader = FileDownloader.new(client: @client)
-        downloader.download(destination, options.merge(bucket: bucket, key: key))
+        download_opts = options.merge(bucket: bucket, key: key)
+        executor = @executor || DefaultExecutor.new(max_threads: download_opts.delete(:thread_count))
+        downloader = FileDownloader.new(client: @client, executor: executor)
+        downloader.download(destination, download_opts)
+        executor.shutdown unless @executor
         true
       end
 
@@ -139,7 +186,7 @@ def download_file(destination, bucket:, key:, **options)
       #   A file on the local file system that will be uploaded. This can either be a `String` or `Pathname` to the
       #   file, an open `File` object, or an open `Tempfile` object. If you pass an open `File` or `Tempfile` object,
       #   then you are responsible for closing it after the upload completes. When using an open Tempfile, rewind it
-      #   before  uploading or else the object will be empty.
+      #   before uploading or else the object will be empty.
       #
       # @param [String] bucket
       #   The name of the S3 bucket to upload to.
@@ -156,15 +203,14 @@ def download_file(destination, bucket:, key:, **options)
       #   Files larger han or equal to `:multipart_threshold` are uploaded using the S3 multipart upload APIs.
       #   Default threshold is `100MB`.
       #
-      # @option options [Integer] :thread_count (10)
-      #    The number of parallel multipart uploads. This option is not used if the file is smaller than
-      #    `:multipart_threshold`.
+      # @option options [Integer] :thread_count (10) Customize threads used in the multipart upload.
+      #   Only used when no custom executor is provided (creates {DefaultExecutor} with the given thread count).
       #
       # @option options [Proc] :progress_callback (nil)
       #   A Proc that will be called when each chunk of the upload is sent.
       #   It will be invoked with `[bytes_read]` and  `[total_sizes]`.
       #
-      # @raise [MultipartUploadError] If an file is being uploaded in parts, and the upload can not be completed,
+      # @raise [MultipartUploadError] If a file is being uploaded in parts, and the upload can not be completed,
       #   then the upload is aborted and this error is raised.  The raised error has a `#errors` method that
       #   returns the failures that caused the upload to be aborted.
       #
@@ -175,13 +221,16 @@ def download_file(destination, bucket:, key:, **options)
       # @see Client#complete_multipart_upload
       # @see Client#upload_part
       def upload_file(source, bucket:, key:, **options)
-        uploading_options = options.dup
+        upload_opts = options.merge(bucket: bucket, key: key)
+        executor = @executor || DefaultExecutor.new(max_threads: upload_opts.delete(:thread_count))
         uploader = FileUploader.new(
-          multipart_threshold: uploading_options.delete(:multipart_threshold),
-          client: @client
+          multipart_threshold: upload_opts.delete(:multipart_threshold),
+          client: @client,
+          executor: executor
         )
-        response = uploader.upload(source, uploading_options.merge(bucket: bucket, key: key))
+        response = uploader.upload(source, upload_opts)
         yield response if block_given?
+        executor.shutdown unless @executor
         true
       end
 
@@ -217,7 +266,8 @@ def upload_file(source, bucket:, key:, **options)
       #   {Client#upload_part} can be provided.
       #
       # @option options [Integer] :thread_count (10)
-      #   The number of parallel multipart uploads.
+      #   The number of parallel multipart uploads. Only used when no custom executor is provided (creates
+      #   {DefaultExecutor} with the given thread count). An additional thread is used internally for task coordination.
       #
       # @option options [Boolean] :tempfile (false)
       #   Normally read data is stored in memory when building the parts in order to complete the underlying
@@ -237,14 +287,16 @@ def upload_file(source, bucket:, key:, **options)
       # @see Client#complete_multipart_upload
       # @see Client#upload_part
       def upload_stream(bucket:, key:, **options, &block)
-        uploading_options = options.dup
+        upload_opts = options.merge(bucket: bucket, key: key)
+        executor = @executor || DefaultExecutor.new(max_threads: upload_opts.delete(:thread_count))
         uploader = MultipartStreamUploader.new(
           client: @client,
-          thread_count: uploading_options.delete(:thread_count),
-          tempfile: uploading_options.delete(:tempfile),
-          part_size: uploading_options.delete(:part_size)
+          executor: executor,
+          tempfile: upload_opts.delete(:tempfile),
+          part_size: upload_opts.delete(:part_size)
         )
-        uploader.upload(uploading_options.merge(bucket: bucket, key: key), &block)
+        uploader.upload(upload_opts, &block)
+        executor.shutdown unless @executor
         true
       end
     end
diff --git a/gems/aws-sdk-s3/spec/default_executor_spec.rb b/gems/aws-sdk-s3/spec/default_executor_spec.rb
new file mode 100644
index 00000000000..ffb6974d096
--- /dev/null
+++ b/gems/aws-sdk-s3/spec/default_executor_spec.rb
@@ -0,0 +1,68 @@
+# frozen_string_literal: true
+
+require_relative 'spec_helper'
+
+module Aws
+  module S3
+    describe DefaultExecutor do
+      let(:subject) { DefaultExecutor.new }
+
+      describe '#post' do
+        it 'executes a block with arguments' do
+          queue = Queue.new
+          subject.post('hello') { |arg| queue << arg }
+          expect(queue.pop).to eq('hello')
+        end
+
+        it 'returns true when a task is submitted' do
+          expect(subject.post('hello') { |_arg| }).to be(true)
+        end
+
+        it 'raises when executor is shutdown' do
+          subject.shutdown
+          expect { subject.post }.to raise_error(RuntimeError)
+        end
+      end
+
+      describe '#shutdown' do
+        it 'waits for running tasks to be complete' do
+          result = nil
+          subject.post { result = true }
+          expect(subject.shutdown).to be(true)
+          expect(result).to be(true)
+        end
+
+        it 'kills threads after timeout' do
+          started = Queue.new
+          counter = 0
+          subject.post do
+            counter += 1
+            started << 'work started'
+            sleep 1
+            counter += 1
+          end
+          started.pop
+          expect(subject.shutdown(0.01)).to be(true)
+          expect(counter).to eq(1)
+        end
+      end
+
+      describe '#kill' do
+        it 'stops all threads immediately and returns true' do
+          started = Queue.new
+          counter = 0
+          subject.post do
+            counter += 1
+            started << 'work started'
+            sleep 1
+            counter += 1
+          end
+          started.pop
+          result = subject.kill
+          expect(result).to be(true)
+          expect(counter).to eq(1)
+        end
+      end
+    end
+  end
+end
diff --git a/gems/aws-sdk-s3/spec/file_downloader_spec.rb b/gems/aws-sdk-s3/spec/file_downloader_spec.rb
index 0f3dd8fb2cf..a596c1fa6ba 100644
--- a/gems/aws-sdk-s3/spec/file_downloader_spec.rb
+++ b/gems/aws-sdk-s3/spec/file_downloader_spec.rb
@@ -7,7 +7,7 @@ module Aws
   module S3
     describe FileDownloader do
       let(:client) { S3::Client.new(stub_responses: true) }
-      let(:subject) { FileDownloader.new(client: client) }
+      let(:subject) { FileDownloader.new(client: client, executor: DefaultExecutor.new) }
       let(:tmpdir) { Dir.tmpdir }
 
       describe '#initialize' do
@@ -198,7 +198,6 @@ module S3
 
           it 'raises when checksum validation fails on multipart object' do
             client.stub_responses(:get_object, { body: 'body', checksum_sha1: 'invalid' })
-            expect(Thread).to receive(:new).and_yield.and_return(double(value: nil))
             expect { subject.download(path, parts_params) }.to raise_error(Aws::Errors::ChecksumError)
           end
 
@@ -208,7 +207,6 @@ module S3
               expect(ctx.params[:if_match]).to eq('test-etag')
               'PreconditionFailed'
             })
-            expect(Thread).to receive(:new).and_yield.and_return(double(value: nil))
             expect { subject.download(path, range_params.merge(chunk_size: one_meg, mode: 'get_range')) }
               .to raise_error(Aws::S3::Errors::PreconditionFailed)
           end
@@ -219,8 +217,6 @@ module S3
               expect(ctx.params[:if_match]).to eq('test-etag')
               'PreconditionFailed'
             })
-
-            expect(Thread).to receive(:new).and_yield.and_return(double(value: nil))
             expect { subject.download(path, parts_params) }.to raise_error(Aws::S3::Errors::PreconditionFailed)
           end
 
@@ -246,7 +242,6 @@ module S3
 
           it 'raises when range validation fails' do
             client.stub_responses(:get_object, { body: 'body', content_range: 'bytes 0-3/4' })
-            expect(Thread).to receive(:new).and_yield.and_return(double(value: nil))
             expect { subject.download(path, range_params.merge(mode: 'get_range', chunk_size: one_meg)) }
               .to raise_error(Aws::S3::MultipartDownloadError)
           end
@@ -263,7 +258,6 @@ module S3
               responses[context.params[:range]]
             })
 
-            expect(Thread).to receive(:new).and_yield.and_return(double(value: nil))
             expect { subject.download(path, range_params.merge(chunk_size: 5 * one_meg, mode: 'get_range')) }
               .to raise_error(Aws::S3::MultipartDownloadError)
             expect(File.exist?(path)).to be(true)
diff --git a/gems/aws-sdk-s3/spec/file_uploader_spec.rb b/gems/aws-sdk-s3/spec/file_uploader_spec.rb
index 64dbaf7cdd2..dc3ab94b052 100644
--- a/gems/aws-sdk-s3/spec/file_uploader_spec.rb
+++ b/gems/aws-sdk-s3/spec/file_uploader_spec.rb
@@ -81,12 +81,6 @@ module S3
 
           subject.upload(ten_meg_file.path, params)
         end
-
-        it 'does not fail when given :thread_count' do
-          expect(client).to receive(:put_object).with(params.merge(body: ten_meg_file))
-
-          subject.upload(ten_meg_file, params.merge(thread_count: 1))
-        end
       end
     end
   end
diff --git a/gems/aws-sdk-s3/spec/multipart_file_uploader_spec.rb b/gems/aws-sdk-s3/spec/multipart_file_uploader_spec.rb
index 82e147986e3..0d2cd2fae4f 100644
--- a/gems/aws-sdk-s3/spec/multipart_file_uploader_spec.rb
+++ b/gems/aws-sdk-s3/spec/multipart_file_uploader_spec.rb
@@ -7,7 +7,7 @@ module Aws
   module S3
     describe MultipartFileUploader do
       let(:client) { S3::Client.new(stub_responses: true) }
-      let(:subject) { MultipartFileUploader.new(client: client) }
+      let(:subject) { MultipartFileUploader.new(client: client, executor: DefaultExecutor.new) }
       let(:params) { { bucket: 'bucket', key: 'key' } }
 
       describe '#initialize' do
@@ -85,7 +85,6 @@ module S3
         end
 
         it 'reports progress for multipart uploads' do
-          allow(Thread).to receive(:new).and_yield.and_return(double(value: nil))
           client.stub_responses(:create_multipart_upload, upload_id: 'id')
           client.stub_responses(:complete_multipart_upload)
           expect(client).to receive(:upload_part).exactly(24).times do |args|
@@ -127,10 +126,6 @@ module S3
         end
 
         it 'reports when it is unable to abort a failed multipart upload' do
-          allow(Thread).to receive(:new) do |_, &block|
-            double(value: block.call)
-          end
-
           client.stub_responses(
             :upload_part,
             [
diff --git a/gems/aws-sdk-s3/spec/multipart_stream_uploader_spec.rb b/gems/aws-sdk-s3/spec/multipart_stream_uploader_spec.rb
index e7c2860f5a7..6262a42b560 100644
--- a/gems/aws-sdk-s3/spec/multipart_stream_uploader_spec.rb
+++ b/gems/aws-sdk-s3/spec/multipart_stream_uploader_spec.rb
@@ -7,7 +7,7 @@ module Aws
   module S3
     describe MultipartStreamUploader do
       let(:client) { S3::Client.new(stub_responses: true) }
-      let(:subject) { MultipartStreamUploader.new(client: client) }
+      let(:subject) { MultipartStreamUploader.new(client: client, executor: DefaultExecutor.new) }
       let(:params) { { bucket: 'bucket', key: 'key' } }
       let(:one_mb) { '.' * 1024 * 1024 }
       let(:seventeen_mb) { one_mb * 17 }
@@ -50,7 +50,6 @@ module S3
             }
           )
           expect(client).to receive(:complete_multipart_upload).with(expected_params).once
-
           subject.upload(params.merge(content_type: 'text/plain')) { |write_stream| write_stream << seventeen_mb }
         end
 
@@ -155,7 +154,7 @@ module S3
         end
 
         context 'when tempfile is true' do
-          let(:subject) { MultipartStreamUploader.new(client: client, tempfile: true) }
+          let(:subject) { MultipartStreamUploader.new(client: client, tempfile: true, executor: DefaultExecutor.new) }
 
           it 'uses multipart APIs' do
             client.stub_responses(:create_multipart_upload, upload_id: 'id')
diff --git a/gems/aws-sdk-s3/spec/object/upload_stream_spec.rb b/gems/aws-sdk-s3/spec/object/upload_stream_spec.rb
index 269b91746eb..d75b58a7515 100644
--- a/gems/aws-sdk-s3/spec/object/upload_stream_spec.rb
+++ b/gems/aws-sdk-s3/spec/object/upload_stream_spec.rb
@@ -27,9 +27,9 @@ module S3
 
         it 'respects the thread_count option' do
           custom_thread_count = 20
-          expect(Thread).to receive(:new).exactly(custom_thread_count).times.and_return(double(value: nil))
           client.stub_responses(:create_multipart_upload, upload_id: 'id')
           client.stub_responses(:complete_multipart_upload)
+          expect(DefaultExecutor).to receive(:new).with(max_threads: custom_thread_count).and_call_original
           subject.upload_stream(thread_count: custom_thread_count) { |_write_stream| }
         end