diff --git a/AUTHORS.rst b/AUTHORS.rst
index 5bcd74c943b..2fe75be08e1 100644
--- a/AUTHORS.rst
+++ b/AUTHORS.rst
@@ -72,6 +72,7 @@ Contributors
 * Joel Wurtz -- cellspanning support in LaTeX
 * John Waltman -- Texinfo builder
 * Jon Dufresne -- modernisation
+* Jorge Marques -- unique ids in singlehtml
 * Josip Dzolonga -- coverage builder
 * Juan Luis Cano Rodríguez -- new tutorial (2021)
 * Julien Palard -- Colspan and rowspan in text builder
diff --git a/CHANGES.rst b/CHANGES.rst
index 64f94e14ec3..7093ff9ef40 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -66,6 +66,9 @@ Features added
   Patch by Adam Turner.
 * #13805: LaTeX: add support for ``fontawesome7`` package.
   Patch by Jean-François B.
+* #13739: singlehtml builder: make all ids unique by appending the docname,
+  matching ``sphinx/environment/adapters/toctree.py``'s ``_resolve_toctree()``
+  format. E.g., ``id3`` becomes ``document-path/to/doc#id3``.
 
 Bugs fixed
 ----------
diff --git a/sphinx/builders/singlehtml.py b/sphinx/builders/singlehtml.py
index 1888f6679d1..9f66d8db318 100644
--- a/sphinx/builders/singlehtml.py
+++ b/sphinx/builders/singlehtml.py
@@ -88,6 +88,22 @@ def _get_local_toctree(
         )
         return self.render_partial(toctree)['fragment']
 
+    def prefix_ids_with_docname(self, tree: nodes.document) -> None:
+        # Append docname to refids and ids using format document-<docname>#<id>.
+        # Compensates for loss of the pathname section of the href, that
+        # ensures uniqueness in the html builder.
+        for node in tree.findall(nodes.Element):
+            doc = node.document
+            if doc is None:
+                continue
+            env = doc.settings.env
+            if 'refid' in node or 'ids' in node:
+                docname = env.path2doc(doc['source'])
+            if 'refid' in node:
+                node['refid'] = 'document-' + docname + '#' + node['refid']
+            if 'ids' in node:
+                node['ids'] = ['document-' + docname + '#' + id for id in node['ids']]
+
     def assemble_doctree(self) -> nodes.document:
         master = self.config.root_doc
         tree = self.env.get_doctree(master)
@@ -95,14 +111,16 @@ def assemble_doctree(self) -> nodes.document:
         tree = inline_all_toctrees(self, set(), master, tree, darkgreen, [master])
         tree['docname'] = master
         self.env.resolve_references(tree, master, self)
+        self.prefix_ids_with_docname(tree)
         return tree
 
     def assemble_toc_secnumbers(self) -> dict[str, dict[str, tuple[int, ...]]]:
         # Assemble toc_secnumbers to resolve section numbers on SingleHTML.
         # Merge all secnumbers to single secnumber.
         #
-        # Note: current Sphinx has refid confliction in singlehtml mode.
-        #       To avoid the problem, it replaces key of secnumbers to
+        # Note: current Sphinx patches refid with docname to avoid confliction
+        #       in singlehtml mode.
+        #       To match the patch, it replaces key of secnumbers to
         #       tuple of docname and refid.
         #
         #       There are related codes in inline_all_toctres() and
@@ -110,7 +128,7 @@ def assemble_toc_secnumbers(self) -> dict[str, dict[str, tuple[int, ...]]]:
         new_secnumbers: dict[str, tuple[int, ...]] = {}
         for docname, secnums in self.env.toc_secnumbers.items():
             for id, secnum in secnums.items():
-                alias = f'{docname}/{id}'
+                alias = f'document-{docname}{id}'
                 new_secnumbers[alias] = secnum
 
         return {self.config.root_doc: new_secnumbers}
@@ -121,8 +139,9 @@ def assemble_toc_fignumbers(
         # Assemble toc_fignumbers to resolve figure numbers on SingleHTML.
         # Merge all fignumbers to single fignumber.
         #
-        # Note: current Sphinx has refid confliction in singlehtml mode.
-        #       To avoid the problem, it replaces key of secnumbers to
+        # Note: current Sphinx patches refid with docname to avoid confliction
+        #       in singlehtml mode.
+        #       To match the patch, it replaces key of secnumbers to
         #       tuple of docname and refid.
         #
         #       There are related codes in inline_all_toctres() and
@@ -131,9 +150,10 @@ def assemble_toc_fignumbers(
         # {'foo': {'figure': {'id2': (2,), 'id1': (1,)}}, 'bar': {'figure': {'id1': (3,)}}}
         for docname, fignumlist in self.env.toc_fignumbers.items():
             for figtype, fignums in fignumlist.items():
-                alias = f'{docname}/{figtype}'
+                alias = f'document-{docname}#{figtype}'
                 new_fignumbers.setdefault(alias, {})
                 for id, fignum in fignums.items():
+                    id = f'document-{docname}#{id}'
                     new_fignumbers[alias][id] = fignum
 
         return {self.config.root_doc: new_fignumbers}
diff --git a/sphinx/writers/html5.py b/sphinx/writers/html5.py
index bbcd247e33c..7d717c68ff6 100644
--- a/sphinx/writers/html5.py
+++ b/sphinx/writers/html5.py
@@ -395,10 +395,10 @@ def get_secnumber(self, node: Element) -> tuple[int, ...] | None:
         if isinstance(node.parent, nodes.section):
             if self.builder.name == 'singlehtml':
                 docname = self.docnames[-1]
-                anchorname = f'{docname}/#{node.parent["ids"][0]}'
+                anchorname = node.parent['ids'][0]
                 if anchorname not in self.builder.secnumbers:
                     # try first heading which has no anchor
-                    anchorname = f'{docname}/'
+                    anchorname = f'document-{docname}'
             else:
                 anchorname = '#' + node.parent['ids'][0]
                 if anchorname not in self.builder.secnumbers:
@@ -420,7 +420,7 @@ def add_secnumber(self, node: Element) -> None:
     def add_fignumber(self, node: Element) -> None:
         def append_fignumber(figtype: str, figure_id: str) -> None:
             if self.builder.name == 'singlehtml':
-                key = f'{self.docnames[-1]}/{figtype}'
+                key = f'document-{self.docnames[-1]}#{figtype}'
             else:
                 key = figtype
 
diff --git a/tests/roots/test-tocdepth/bar.rst b/tests/roots/test-tocdepth/bar.rst
index d70dec90dd3..2bb869e93c5 100644
--- a/tests/roots/test-tocdepth/bar.rst
+++ b/tests/roots/test-tocdepth/bar.rst
@@ -25,3 +25,8 @@ Bar B1
 
 should be 2.2.1
 
+FooBar B1
+---------
+
+should be 2.2.2
+
diff --git a/tests/roots/test-tocdepth/foo.rst b/tests/roots/test-tocdepth/foo.rst
index 61fd539ffea..4834cb6eb14 100644
--- a/tests/roots/test-tocdepth/foo.rst
+++ b/tests/roots/test-tocdepth/foo.rst
@@ -24,3 +24,8 @@ Foo B1
 
 should be 1.2.1
 
+FooBar B1
+---------
+
+should be 1.2.2
+
diff --git a/tests/test_builders/test_build_html_tocdepth.py b/tests/test_builders/test_build_html_tocdepth.py
index 0fe83e0ff34..1c02b0da415 100644
--- a/tests/test_builders/test_build_html_tocdepth.py
+++ b/tests/test_builders/test_build_html_tocdepth.py
@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, cast
 
 import pytest
 
@@ -12,7 +12,7 @@
 if TYPE_CHECKING:
     from collections.abc import Callable
     from pathlib import Path
-    from xml.etree.ElementTree import ElementTree
+    from xml.etree.ElementTree import Element, ElementTree
 
     from sphinx.testing.util import SphinxTestApp
 
@@ -134,3 +134,17 @@ def test_tocdepth_singlehtml(
 ) -> None:
     app.build()
     check_xpath(cached_etree_parse(app.outdir / 'index.html'), 'index.html', *expect)
+
+
+@pytest.mark.sphinx('singlehtml', testroot='tocdepth')
+@pytest.mark.test_params(shared_result='test_build_html_tocdepth')
+def test_unique_ids_singlehtml(
+    app: SphinxTestApp,
+    cached_etree_parse: Callable[[Path], ElementTree],
+) -> None:
+    app.build()
+    tree = cached_etree_parse(app.outdir / 'index.html')
+    root = cast('Element', tree.getroot())
+
+    ids = [el.attrib['id'] for el in root.findall('.//*[@id]')]
+    assert len(ids) == len(set(ids))