diff --git a/CHANGES b/CHANGES
index 14534b096..b6f8a8d5f 100644
--- a/CHANGES
+++ b/CHANGES
@@ -27,6 +27,7 @@ Deprecated
   generate_autosummary_docs()``
 * The ``ignore`` argument of ``sphinx.util.docstring.prepare_docstring()``
 * ``sphinx.ext.autosummary.generate.AutosummaryRenderer.exists()``
+* ``sphinx.util.rpartition()``
 
 Features added
 --------------
diff --git a/doc/extdev/deprecated.rst b/doc/extdev/deprecated.rst
index 9d05f894b..7e59184ec 100644
--- a/doc/extdev/deprecated.rst
+++ b/doc/extdev/deprecated.rst
@@ -78,6 +78,11 @@ The following is a list of deprecated interfaces.
      - 5.0
      - N/A
 
+   * - ``sphinx.util.rpartition()``
+     - 3.1
+     - 5.0
+     - ``str.rpartition()``
+
    * - ``desc_signature['first']``
      -
      - 3.0
diff --git a/sphinx/search/__init__.py b/sphinx/search/__init__.py
index f9bbe0590..74fbaade9 100644
--- a/sphinx/search/__init__.py
+++ b/sphinx/search/__init__.py
@@ -23,7 +23,7 @@ from sphinx import package_dir
 from sphinx.deprecation import RemovedInSphinx40Warning
 from sphinx.environment import BuildEnvironment
 from sphinx.search.jssplitter import splitter_code
-from sphinx.util import jsdump, rpartition
+from sphinx.util import jsdump
 
 if False:
     # For type annotation
@@ -333,7 +333,7 @@ class IndexBuilder:
                     continue
                 fullname = html.escape(fullname)
                 dispname = html.escape(dispname)
-                prefix, name = rpartition(dispname, '.')
+                prefix, _, name = dispname.rpartition('.')
                 pdict = rv.setdefault(prefix, {})
                 try:
                     typeindex = otypes[domainname, type]
diff --git a/sphinx/util/__init__.py b/sphinx/util/__init__.py
index 66f5fe060..ca9bb028d 100644
--- a/sphinx/util/__init__.py
+++ b/sphinx/util/__init__.py
@@ -28,7 +28,7 @@ from time import mktime, strptime
 from typing import Any, Callable, Dict, IO, Iterable, Iterator, List, Pattern, Set, Tuple
 from urllib.parse import urlsplit, urlunsplit, quote_plus, parse_qsl, urlencode
 
-from sphinx.deprecation import RemovedInSphinx40Warning
+from sphinx.deprecation import RemovedInSphinx40Warning, RemovedInSphinx50Warning
 from sphinx.errors import (
     PycodeError, SphinxParallelError, ExtensionError, FiletypeNotFoundError
 )
@@ -497,6 +497,7 @@ class attrdict(dict):
 
 def rpartition(s: str, t: str) -> Tuple[str, str]:
     """Similar to str.rpartition from 2.5, but doesn't return the separator."""
+    warnings.warn('rpartition() is now deprecated.', RemovedInSphinx50Warning, stacklevel=2)
     i = s.rfind(t)
     if i != -1:
         return s[:i], s[i + len(t):]