diff --git a/AUTHORS.rst b/AUTHORS.rst
index 5bcd74c943b..614b4b4aacd 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 -- singlehtml unique section ids
 * 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 b5812ef2f23..4e3b7a5a089 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -116,6 +116,9 @@ Bugs fixed
 * #10785: Autodoc: Allow type aliases defined in the project to be properly
   cross-referenced when used as type annotations. This makes it possible
   for objects documented as ``:py:data:`` to be hyperlinked in function signatures.
+* #13738: singlehtml builder: make section 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``.
 
 Testing
 -------
diff --git a/sphinx/builders/singlehtml.py b/sphinx/builders/singlehtml.py
index 1888f6679d1..4fd04b05fc0 100644
--- a/sphinx/builders/singlehtml.py
+++ b/sphinx/builders/singlehtml.py
@@ -110,7 +110,7 @@ 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'{docname}{id}'
                 new_secnumbers[alias] = secnum
 
         return {self.config.root_doc: new_secnumbers}
diff --git a/sphinx/writers/html5.py b/sphinx/writers/html5.py
index bbcd247e33c..14b5eff2914 100644
--- a/sphinx/writers/html5.py
+++ b/sphinx/writers/html5.py
@@ -17,7 +17,7 @@
 from sphinx.util.images import get_image_size
 
 if TYPE_CHECKING:
-    from docutils.nodes import Element, Node, Text
+    from docutils.nodes import Element, Node, Text, section
 
     from sphinx.builders import Builder
     from sphinx.builders.html import StandaloneHTMLBuilder
@@ -395,10 +395,11 @@ 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]}'
+                # Remove document-
+                anchorname = node.parent['ids'][0][9:]
                 if anchorname not in self.builder.secnumbers:
                     # try first heading which has no anchor
-                    anchorname = f'{docname}/'
+                    anchorname = docname
             else:
                 anchorname = '#' + node.parent['ids'][0]
                 if anchorname not in self.builder.secnumbers:
@@ -497,6 +498,15 @@ def depart_term(self, node: Element) -> None:
 
             self.body.append('</dt>')
 
+    def visit_section(self, node: section) -> None:
+        if self.builder.name == 'singlehtml' and node['ids']:
+            docname = self.docnames[-1]
+            node['ids'][0] = 'document-' + docname + '#' + node['ids'][0]
+        super().visit_section(node)
+
+    def depart_section(self, node: section) -> None:
+        super().depart_section(node)
+
     # overwritten
     def visit_title(self, node: nodes.title) -> None:
         if (
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))