diff --git a/Doc/library/mimetypes.rst b/Doc/library/mimetypes.rst
index 0facacd50fd..eed9acb92e0 100644
--- a/Doc/library/mimetypes.rst
+++ b/Doc/library/mimetypes.rst
@@ -129,10 +129,18 @@ behavior of the module.
    Add a mapping from the MIME type *type* to the extension *ext*. When the
    extension is already known, the new type will replace the old one. When the type
    is already known the extension will be added to the list of known extensions.
+   Valid extensions are empty or start with a ``'.'``.
 
    When *strict* is ``True`` (the default), the mapping will be added to the
    official MIME types, otherwise to the non-standard ones.
 
+   .. deprecated:: 3.14
+      *ext* values that do not start with ``'.'`` are deprecated.
+
+   .. versionchanged:: next
+      *ext* now must start with ``'.'``. Otherwise :exc:`ValueError` is raised.
+
+
 
 .. data:: inited
 
diff --git a/Doc/whatsnew/3.16.rst b/Doc/whatsnew/3.16.rst
index 6d91d53f478..f79f976ee55 100644
--- a/Doc/whatsnew/3.16.rst
+++ b/Doc/whatsnew/3.16.rst
@@ -127,6 +127,13 @@ logging
   and scheduled for removal in Python 3.16. Define handlers with the *stream*
   argument instead.
 
+mimetypes
+---------
+
+* Valid extensions start with a '.' or are empty for
+  :meth:`mimetypes.MimeTypes.add_type`.
+  Undotted extensions now raise a :exc:`ValueError`.
+
 symtable
 --------
 
diff --git a/Lib/mimetypes.py b/Lib/mimetypes.py
index 6d9278bccf9..15e8c0a437b 100644
--- a/Lib/mimetypes.py
+++ b/Lib/mimetypes.py
@@ -93,14 +93,7 @@ def add_type(self, type, ext, strict=True):
         Valid extensions are empty or start with a '.'.
         """
         if ext and not ext.startswith('.'):
-            from warnings import _deprecated
-
-            _deprecated(
-                "Undotted extensions",
-                "Using undotted extensions is deprecated and "
-                "will raise a ValueError in Python {remove}",
-                remove=(3, 16),
-            )
+            raise ValueError(f"Extension {ext!r} must start with '.'")
 
         if not type:
             return
diff --git a/Lib/test/test_mimetypes.py b/Lib/test/test_mimetypes.py
index 2d618081521..607aff8418e 100644
--- a/Lib/test/test_mimetypes.py
+++ b/Lib/test/test_mimetypes.py
@@ -389,9 +389,12 @@ def test_added_types_are_used(self):
         mime_type, _ = mimetypes.guess_type('test.myext')
         self.assertEqual(mime_type, 'testing/type')
 
-    def test_add_type_with_undotted_extension_deprecated(self):
-        with self.assertWarns(DeprecationWarning):
+    def test_add_type_with_undotted_extension_not_supported(self):
+        msg = "Extension 'undotted' must start with '.'"
+        with self.assertRaisesRegex(ValueError, msg):
             mimetypes.add_type("testing/type", "undotted")
+        with self.assertRaisesRegex(ValueError, msg):
+            mimetypes.add_type("", "undotted")
 
 
 @unittest.skipUnless(sys.platform.startswith("win"), "Windows only")
diff --git a/Misc/NEWS.d/next/Library/2026-05-12-14-44-39.gh-issue-149720.ccsoW-.rst b/Misc/NEWS.d/next/Library/2026-05-12-14-44-39.gh-issue-149720.ccsoW-.rst
new file mode 100644
index 00000000000..f16d8770d74
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-05-12-14-44-39.gh-issue-149720.ccsoW-.rst
@@ -0,0 +1 @@
+Remove support for undotted *ext* in :meth:`mimetypes.MimeTypes.add_type`.