diff --git a/CHANGELOG.md b/CHANGELOG.md
index 728da8c90..82f19d6b4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -57,6 +57,7 @@ You can find and compare releases at the [GitHub release page](https://github.co
 - Allow lazy enum values
 - Make `Node` implement `JsonSerializable`
 - Add SDL validation rule `UniqueTypeNames` (#998)
+- Add support for SDL validation to `KnownTypeNames` rule (#999)
 
 ### Optimized
 
diff --git a/src/Utils/BuildSchema.php b/src/Utils/BuildSchema.php
index 5e4ee55de..8e08b4f45 100644
--- a/src/Utils/BuildSchema.php
+++ b/src/Utils/BuildSchema.php
@@ -212,26 +212,17 @@ static function (string $typeName): Type {
     }
 
     /**
-     * @throws Error
-     *
      * @return array<string, string>
      */
     private function getOperationTypes(SchemaDefinitionNode $schemaDef): array
     {
-        $opTypes = [];
-
+        /** @var array<string, string> $operationTypes */
+        $operationTypes = [];
         foreach ($schemaDef->operationTypes as $operationType) {
-            $typeName = $operationType->type->name->value;
-            $operation = $operationType->operation;
-
-            if (! isset($this->nodeMap[$typeName])) {
-                throw new Error('Specified ' . $operation . ' type "' . $typeName . '" not found in document.');
-            }
-
-            $opTypes[$operation] = $typeName;
+            $operationTypes[$operationType->operation] = $operationType->type->name->value;
         }
 
-        return $opTypes;
+        return $operationTypes;
     }
 
     public static function unknownType(string $typeName): Error
diff --git a/src/Utils/SchemaExtender.php b/src/Utils/SchemaExtender.php
index 9300bf549..1029b1087 100644
--- a/src/Utils/SchemaExtender.php
+++ b/src/Utils/SchemaExtender.php
@@ -614,11 +614,11 @@ public static function extend(
             $typeDefinitionMap,
             static function (string $typeName) use ($schema): Type {
                 $existingType = $schema->getType($typeName);
-                if ($existingType !== null) {
-                    return static::extendNamedType($existingType);
+                if ($existingType === null) {
+                    throw new InvariantViolation('Unknown type: "' . $typeName . '".');
                 }
 
-                throw new Error('Unknown type: "' . $typeName . '". Ensure that this type exists either in the original schema, or is added in a type definition.');
+                return static::extendNamedType($existingType);
             },
             $typeConfigDecorator
         );
diff --git a/src/Validator/DocumentValidator.php b/src/Validator/DocumentValidator.php
index 119be619c..5e41a9166 100644
--- a/src/Validator/DocumentValidator.php
+++ b/src/Validator/DocumentValidator.php
@@ -203,6 +203,7 @@ public static function sdlRules(): array
             LoneSchemaDefinition::class => new LoneSchemaDefinition(),
             UniqueOperationTypes::class => new UniqueOperationTypes(),
             UniqueTypeNames::class => new UniqueTypeNames(),
+            KnownTypeNames::class => new KnownTypeNames(),
             KnownDirectives::class => new KnownDirectives(),
             KnownArgumentNamesOnDirectives::class => new KnownArgumentNamesOnDirectives(),
             UniqueDirectivesPerLocation::class => new UniqueDirectivesPerLocation(),
diff --git a/src/Validator/Rules/KnownTypeNames.php b/src/Validator/Rules/KnownTypeNames.php
index e7a96b527..05c69aca1 100644
--- a/src/Validator/Rules/KnownTypeNames.php
+++ b/src/Validator/Rules/KnownTypeNames.php
@@ -7,45 +7,86 @@
 use GraphQL\Error\Error;
 use GraphQL\Language\AST\NamedTypeNode;
 use GraphQL\Language\AST\NodeKind;
-use GraphQL\Language\Visitor;
-use GraphQL\Language\VisitorOperation;
+use GraphQL\Language\AST\TypeDefinitionNode;
+use GraphQL\Language\AST\TypeSystemDefinitionNode;
+use GraphQL\Language\AST\TypeSystemExtensionNode;
+use GraphQL\Type\Definition\Type;
 use GraphQL\Utils\Utils;
 use GraphQL\Validator\QueryValidationContext;
+use GraphQL\Validator\SDLValidationContext;
+use GraphQL\Validator\ValidationContext;
+use function in_array;
 
 /**
  * Known type names.
  *
  * A GraphQL document is only valid if referenced types (specifically
  * variable definitions and fragment conditions) are defined by the type schema.
+ *
+ * @phpstan-import-type VisitorArray from \GraphQL\Language\Visitor
  */
 class KnownTypeNames extends ValidationRule
 {
     public function getVisitor(QueryValidationContext $context): array
     {
-        $skip = static function (): VisitorOperation {
-            return Visitor::skipNode();
-        };
+        return $this->getASTVisitor($context);
+    }
+
+    public function getSDLVisitor(SDLValidationContext $context): array
+    {
+        return $this->getASTVisitor($context);
+    }
+
+    /**
+     * @phpstan-return VisitorArray
+     */
+    public function getASTVisitor(ValidationContext $context): array
+    {
+        /** @var array<int, string> $definedTypes */
+        $definedTypes = [];
+        foreach ($context->getDocument()->definitions as $def) {
+            if ($def instanceof TypeDefinitionNode) {
+                $definedTypes[] = $def->name->value;
+            }
+        }
+
+        $standardTypeNames = array_keys(Type::getAllBuiltInTypes());
 
         return [
-            // TODO: when validating IDL, re-enable these. Experimental version does not
-            // add unreferenced types, resulting in false-positive errors. Squelched
-            // errors for now.
-            NodeKind::OBJECT_TYPE_DEFINITION => $skip,
-            NodeKind::INTERFACE_TYPE_DEFINITION => $skip,
-            NodeKind::UNION_TYPE_DEFINITION => $skip,
-            NodeKind::INPUT_OBJECT_TYPE_DEFINITION => $skip,
-            NodeKind::NAMED_TYPE => static function (NamedTypeNode $node) use ($context): void {
-                $schema = $context->getSchema();
+            NodeKind::NAMED_TYPE => static function (NamedTypeNode $node, $_1, $parent, $_2, $ancestors) use ($context, $definedTypes, $standardTypeNames): void {
                 $typeName = $node->name->value;
-                $type = $schema->getType($typeName);
-                if ($type !== null) {
+                $schema = $context->getSchema();
+
+                if (in_array($typeName, $definedTypes, true)) {
+                    return;
+                }
+
+                if ($schema !== null && $schema->hasType($typeName)) {
+                    return;
+                }
+
+                $definitionNode = $ancestors[2] ?? $parent;
+                $isSDL = $definitionNode instanceof TypeSystemDefinitionNode || $definitionNode instanceof TypeSystemExtensionNode;
+                if ($isSDL && in_array($typeName, $standardTypeNames, true)) {
                     return;
                 }
 
+                $existingTypesMap = $schema !== null
+                    ? $schema->getTypeMap()
+                    : [];
+                $typeNames = [
+                    ...array_keys($existingTypesMap),
+                    ...$definedTypes,
+                ];
                 $context->reportError(new Error(
                     static::unknownTypeMessage(
                         $typeName,
-                        Utils::suggestionList($typeName, array_keys($schema->getTypeMap()))
+                        Utils::suggestionList(
+                            $typeName,
+                            $isSDL
+                                ? [...$standardTypeNames, ...$typeNames]
+                                : $typeNames
+                        )
                     ),
                     [$node]
                 ));
diff --git a/tests/Utils/BuildSchemaLegacyTest.php b/tests/Utils/BuildSchemaLegacyTest.php
index 6418a8568..1149a3761 100644
--- a/tests/Utils/BuildSchemaLegacyTest.php
+++ b/tests/Utils/BuildSchemaLegacyTest.php
@@ -3,7 +3,6 @@
 namespace GraphQL\Tests\Utils;
 
 use GraphQL\Error\DebugFlag;
-use GraphQL\Error\Error;
 use GraphQL\GraphQL;
 use GraphQL\Language\Parser;
 use GraphQL\Utils\BuildSchema;
@@ -14,7 +13,6 @@
  * Their counterparts have been removed from `buildASTSchema-test.js` and moved elsewhere,
  * but these changes to `graphql-js` haven't been reflected in `graphql-php` yet.
  * TODO align with:
- *   - https://github.com/graphql/graphql-js/commit/3b9ea61f2348215dee755f779caef83df749d2bb
  *   - https://github.com/graphql/graphql-js/commit/64a5c3448a201737f9218856786c51d66f2deabd.
  */
 class BuildSchemaLegacyTest extends TestCase
@@ -145,177 +143,4 @@ interface Character {
         $result = GraphQL::executeQuery($schema, $source, $rootValue);
         self::assertEquals($expected, $result->toArray(DebugFlag::INCLUDE_DEBUG_MESSAGE));
     }
-
-    // Describe: Failures
-
-    /**
-     * @see it('Unknown type referenced')
-     */
-    public function testUnknownTypeReferenced(): void
-    {
-        $this->expectExceptionObject(BuildSchema::unknownType('Bar'));
-        $sdl = '
-            schema {
-              query: Hello
-            }
-            
-            type Hello {
-              bar: Bar
-            }
-        ';
-        $doc = Parser::parse($sdl);
-        $schema = BuildSchema::buildAST($doc);
-        $schema->getTypeMap();
-    }
-
-    /**
-     * @see it('Unknown type in interface list')
-     */
-    public function testUnknownTypeInInterfaceList(): void
-    {
-        $this->expectExceptionObject(BuildSchema::unknownType('Bar'));
-        $sdl = '
-            type Query implements Bar {
-              field: String
-            }
-        ';
-        $doc = Parser::parse($sdl);
-        $schema = BuildSchema::buildAST($doc);
-        $schema->getTypeMap();
-    }
-
-    /**
-     * @see it('Unknown type in union list')
-     */
-    public function testUnknownTypeInUnionList(): void
-    {
-        $this->expectExceptionObject(BuildSchema::unknownType('Bar'));
-        $sdl = '
-            union TestUnion = Bar
-            type Query { testUnion: TestUnion }
-        ';
-        $doc = Parser::parse($sdl);
-        $schema = BuildSchema::buildAST($doc);
-        $schema->getTypeMap();
-    }
-
-    /**
-     * @see it('Unknown query type')
-     */
-    public function testUnknownQueryType(): void
-    {
-        $this->expectException(Error::class);
-        $this->expectExceptionMessage('Specified query type "Wat" not found in document.');
-        $sdl = '
-            schema {
-              query: Wat
-            }
-            
-            type Hello {
-              str: String
-            }
-        ';
-        $doc = Parser::parse($sdl);
-        BuildSchema::buildAST($doc);
-    }
-
-    /**
-     * @see it('Unknown mutation type')
-     */
-    public function testUnknownMutationType(): void
-    {
-        $this->expectException(Error::class);
-        $this->expectExceptionMessage('Specified mutation type "Wat" not found in document.');
-        $sdl = '
-            schema {
-              query: Hello
-              mutation: Wat
-            }
-            
-            type Hello {
-              str: String
-            }
-        ';
-        $doc = Parser::parse($sdl);
-        BuildSchema::buildAST($doc);
-    }
-
-    /**
-     * @see it('Unknown subscription type')
-     */
-    public function testUnknownSubscriptionType(): void
-    {
-        $this->expectException(Error::class);
-        $this->expectExceptionMessage('Specified subscription type "Awesome" not found in document.');
-        $sdl = '
-            schema {
-              query: Hello
-              mutation: Wat
-              subscription: Awesome
-            }
-            
-            type Hello {
-              str: String
-            }
-            
-            type Wat {
-              str: String
-            }
-        ';
-        $doc = Parser::parse($sdl);
-        BuildSchema::buildAST($doc);
-    }
-
-    /**
-     * @see it('Does not consider directive names')
-     */
-    public function testDoesNotConsiderDirectiveNames(): void
-    {
-        $sdl = '
-          schema {
-            query: Foo
-          }
-    
-          directive @Foo on QUERY
-        ';
-        $doc = Parser::parse($sdl);
-        $this->expectExceptionMessage('Specified query type "Foo" not found in document.');
-        BuildSchema::build($doc);
-    }
-
-    /**
-     * @see it('Does not consider operation names')
-     */
-    public function testDoesNotConsiderOperationNames(): void
-    {
-        $this->expectException(Error::class);
-        $this->expectExceptionMessage('Specified query type "Foo" not found in document.');
-        $sdl = '
-            schema {
-              query: Foo
-            }
-            
-            query Foo { field }
-        ';
-        $doc = Parser::parse($sdl);
-        BuildSchema::buildAST($doc);
-    }
-
-    /**
-     * @see it('Does not consider fragment names')
-     */
-    public function testDoesNotConsiderFragmentNames(): void
-    {
-        $this->expectException(Error::class);
-        $this->expectExceptionMessage('Specified query type "Foo" not found in document.');
-        $sdl = '
-            schema {
-              query: Foo
-            }
-            
-            fragment Foo on Type { field }
-        ';
-        $doc = Parser::parse($sdl);
-        BuildSchema::buildAST($doc);
-    }
 }
diff --git a/tests/Utils/SchemaExtenderLegacyTest.php b/tests/Utils/SchemaExtenderLegacyTest.php
index 38c208a68..47efdb5ce 100644
--- a/tests/Utils/SchemaExtenderLegacyTest.php
+++ b/tests/Utils/SchemaExtenderLegacyTest.php
@@ -29,7 +29,6 @@
  * but these changes to `graphql-js` haven't been reflected in `graphql-php` yet.
  * TODO align with:
  *   - https://github.com/graphql/graphql-js/commit/c1745228b2ae5ec89b8de36ea766d544607e21ea
- *   - https://github.com/graphql/graphql-js/commit/3b9ea61f2348215dee755f779caef83df749d2bb
  *   - https://github.com/graphql/graphql-js/commit/e6a3f08cc92594f68a6e61d3d4b46a6d279f845e.
  */
 class SchemaExtenderLegacyTest extends TestCase
@@ -263,66 +262,6 @@ public function testDoesNotAllowReplacingAnExistingField(): void
         }
     }
 
-    // Validation: add support of SDL to KnownTypeNames
-
-    /**
-     * @see it('does not allow referencing an unknown type')
-     */
-    public function testDoesNotAllowReferencingAnUnknownType(): void
-    {
-        $unknownTypeError = 'Unknown type: "Quix". Ensure that this type exists either in the original schema, or is added in a type definition.';
-
-        $typeSDL = '
-          extend type Bar {
-            quix: Quix
-          }
-        ';
-
-        try {
-            $this->extendTestSchema($typeSDL);
-            self::fail();
-        } catch (Error $error) {
-            self::assertEquals($unknownTypeError, $error->getMessage());
-        }
-
-        $interfaceSDL = '
-          extend interface SomeInterface {
-            quix: Quix
-          }
-        ';
-
-        try {
-            $this->extendTestSchema($interfaceSDL);
-            self::fail();
-        } catch (Error $error) {
-            self::assertEquals($unknownTypeError, $error->getMessage());
-        }
-
-        $unionSDL = '
-          extend union SomeUnion = Quix
-        ';
-
-        try {
-            $this->extendTestSchema($unionSDL);
-            self::fail();
-        } catch (Error $error) {
-            self::assertEquals($unknownTypeError, $error->getMessage());
-        }
-
-        $inputSDL = '
-          extend input SomeInput {
-            quix: Quix
-          }
-        ';
-
-        try {
-            $this->extendTestSchema($inputSDL);
-            self::fail();
-        } catch (Error $error) {
-            self::assertEquals($unknownTypeError, $error->getMessage());
-        }
-    }
-
     // Extract check for possible extensions into a separate rule (#1643)
 
     /**
diff --git a/tests/Validator/KnownTypeNamesTest.php b/tests/Validator/KnownTypeNamesTest.php
index a1ef1f34b..3cbe24ad9 100644
--- a/tests/Validator/KnownTypeNamesTest.php
+++ b/tests/Validator/KnownTypeNamesTest.php
@@ -2,8 +2,6 @@
 
 namespace GraphQL\Tests\Validator;
 
-use GraphQL\Language\SourceLocation;
-use GraphQL\Tests\ErrorHelper;
 use GraphQL\Utils\BuildSchema;
 use GraphQL\Validator\Rules\KnownTypeNames;
 
@@ -28,6 +26,7 @@ public function testKnownTypeNamesAreValid(): void
           pets { ... on Pet { name }, ...PetFields, ... { name } }
         }
       }
+
       fragment PetFields on Pet {
         name
       }
@@ -43,7 +42,7 @@ public function testUnknownTypeNamesAreInvalid(): void
         $this->expectFailsRule(
             new KnownTypeNames(),
             '
-      query Foo($var: [JumbledUpLetters!]!) {
+      query Foo($var: JumbledUpLetters) {
         user(id: 4) {
           name
           pets { ... on Badger { name }, ...PetFields }
@@ -54,9 +53,18 @@ public function testUnknownTypeNamesAreInvalid(): void
       }
         ',
             [
-                $this->unknownType('JumbledUpLetters', [], 2, 24),
-                $this->unknownType('Badger', [], 5, 25),
-                $this->unknownType('Peat', ['Pet', 'Cat'], 8, 29),
+                [
+                    'message' => 'Unknown type "JumbledUpLetters".',
+                    'locations' => [['line' => 2, 'column' => 23]],
+                ],
+                [
+                    'message' => 'Unknown type "Badger".',
+                    'locations' => [['line' => 5, 'column' => 25]],
+                ],
+                [
+                    'message' => 'Unknown type "Peat". Did you mean "Pet" or "Cat"?',
+                    'locations' => [['line' => 8, 'column' => 29]],
+                ],
             ]
         );
     }
@@ -66,7 +74,7 @@ public function testUnknownTypeNamesAreInvalid(): void
      */
     public function testReferencesToStandardScalarsThatAreMissingInSchema(): void
     {
-        self::markTestSkipped('Missing implementation for SDL validation for now');
+        self::markTestSkipped('TODO we differ from graphql-js due to lazy loading, see https://github.com/webonyx/graphql-php/issues/964#issuecomment-945969162');
         $schema = BuildSchema::build('type Query { foo: String }');
         $query = '
       query ($id: ID, $float: Float, $int: Int) {
@@ -78,9 +86,18 @@ public function testReferencesToStandardScalarsThatAreMissingInSchema(): void
             new KnownTypeNames(),
             $query,
             [
-                $this->unknownType('Unknown type "ID".', [], 2, 19),
-                $this->unknownType('Unknown type "Float".', [], 2, 31),
-                $this->unknownType('Unknown type "Int".', [], 2, 44),
+                [
+                    'message' => 'Unknown type "ID".',
+                    'locations' => [['line' => 2, 'column' => 19]],
+                ],
+                [
+                    'message' => 'Unknown type "Float".',
+                    'locations' => [['line' => 2, 'column' => 31]],
+                ],
+                [
+                    'message' => 'Unknown type "Int".',
+                    'locations' => [['line' => 2, 'column' => 44]],
+                ],
             ]
         );
     }
@@ -154,7 +171,6 @@ public function testReferenceTypesDefinedInsideTheSameDocument(): void
      */
     public function testUnknownTypeReferences(): void
     {
-        self::markTestSkipped('Missing implementation for SDL validation for now');
         $this->expectSDLErrorsFromRule(
             new KnownTypeNames(),
             '
@@ -242,7 +258,6 @@ interface SomeInterface {
      */
     public function testDoesNotConsiderNonTypeDefinitions(): void
     {
-        self::markTestSkipped('Missing implementation for SDL validation for now');
         $this->expectSDLErrorsFromRule(
             new KnownTypeNames(),
             '
@@ -319,10 +334,8 @@ public function testReferenceTypesInsideExtensionDocument(): void
      */
     public function testUnknownTypeReferencesInsideExtensionDocument(): void
     {
-        self::markTestSkipped('Missing implementation for SDL validation for now');
         $schema = BuildSchema::build('type A');
         $sdl = '
-        
         type B
 
         type SomeObject implements C {
@@ -355,69 +368,53 @@ interface SomeInterface {
             [
                 [
                     'message' => 'Unknown type "C". Did you mean "A" or "B"?',
-                    'locations' => [['line' => 5, 'column' => 36]],
+                    'locations' => [['line' => 4, 'column' => 36]],
                 ],
                 [
                     'message' => 'Unknown type "D". Did you mean "A", "B", or "ID"?',
-                    'locations' => [['line' => 6, 'column' => 16]],
+                    'locations' => [['line' => 5, 'column' => 16]],
                 ],
                 [
                     'message' => 'Unknown type "E". Did you mean "A" or "B"?',
-                    'locations' => [['line' => 6, 'column' => 20]],
+                    'locations' => [['line' => 5, 'column' => 20]],
                 ],
                 [
                     'message' => 'Unknown type "F". Did you mean "A" or "B"?',
-                    'locations' => [['line' => 9, 'column' => 27]],
+                    'locations' => [['line' => 8, 'column' => 27]],
                 ],
                 [
                     'message' => 'Unknown type "G". Did you mean "A" or "B"?',
-                    'locations' => [['line' => 9, 'column' => 31]],
+                    'locations' => [['line' => 8, 'column' => 31]],
                 ],
                 [
                     'message' => 'Unknown type "H". Did you mean "A" or "B"?',
-                    'locations' => [['line' => 12, 'column' => 16]],
+                    'locations' => [['line' => 11, 'column' => 16]],
                 ],
                 [
                     'message' => 'Unknown type "I". Did you mean "A", "B", or "ID"?',
-                    'locations' => [['line' => 12, 'column' => 20]],
+                    'locations' => [['line' => 11, 'column' => 20]],
                 ],
                 [
                     'message' => 'Unknown type "J". Did you mean "A" or "B"?',
-                    'locations' => [['line' => 16, 'column' => 14]],
+                    'locations' => [['line' => 15, 'column' => 14]],
                 ],
                 [
                     'message' => 'Unknown type "K". Did you mean "A" or "B"?',
-                    'locations' => [['line' => 19, 'column' => 37]],
+                    'locations' => [['line' => 18, 'column' => 37]],
                 ],
                 [
                     'message' => 'Unknown type "L". Did you mean "A" or "B"?',
-                    'locations' => [['line' => 22, 'column' => 18]],
+                    'locations' => [['line' => 21, 'column' => 18]],
                 ],
                 [
                     'message' => 'Unknown type "M". Did you mean "A" or "B"?',
-                    'locations' => [['line' => 23, 'column' => 21]],
+                    'locations' => [['line' => 22, 'column' => 21]],
                 ],
                 [
                     'message' => 'Unknown type "N". Did you mean "A" or "B"?',
-                    'locations' => [['line' => 24, 'column' => 25]],
+                    'locations' => [['line' => 23, 'column' => 25]],
                 ],
             ]
         );
     }
-
-    /**
-     * @param array<string> $suggestedTypes
-     *
-     * @return array{
-     *     message: string,
-     *     locations?: array<int, array{line: int, column: int}>
-     * }
-     */
-    private function unknownType(string $typeName, array $suggestedTypes, int $line, int $column)
-    {
-        return ErrorHelper::create(
-            KnownTypeNames::unknownTypeMessage($typeName, $suggestedTypes),
-            [new SourceLocation($line, $column)]
-        );
-    }
 }