diff --git a/AUTHORS.rst b/AUTHORS.rst
index d08f44875b4..7325c1bc407 100644
--- a/AUTHORS.rst
+++ b/AUTHORS.rst
@@ -71,6 +71,7 @@ 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 791038e5e15..e753bbd040e 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -97,6 +97,9 @@ Bugs fixed
   Patch by Bénédikt Tran.
 * #13712: intersphinx: Don't add "v" prefix to non-numeric versions.
   Patch by Szymon Karpinski.
+* #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 39d7ecea680..aabaaa9a0d1 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 (