diff --git a/Lib/mimetypes.py b/Lib/mimetypes.py
index d7c4e8444f8dec..6c7d343135088e 100644
--- a/Lib/mimetypes.py
+++ b/Lib/mimetypes.py
@@ -94,7 +94,12 @@ def add_type(self, type, ext, strict=True):
         If strict is true, information will be added to
         list of standard types, else to the list of non-standard
         types.
+
+        Valid extensions are empty or start with a '.'.
+        Raise a ValueError for invalid extensions.
         """
+        if ext and not ext.startswith('.'):
+            raise ValueError("Extensions should start with a '.' or be empty")
         self.types_map[strict][ext] = type
         exts = self.types_map_inv[strict].setdefault(type, [])
         if ext not in exts:
diff --git a/Lib/test/test_mimetypes.py b/Lib/test/test_mimetypes.py
index 58f6a4dfae08ba..9bb00c106f4b5c 100644
--- a/Lib/test/test_mimetypes.py
+++ b/Lib/test/test_mimetypes.py
@@ -3,6 +3,7 @@
 import os
 import sys
 import unittest.mock
+import warnings
 
 from test import support
 from test.support import os_helper
@@ -305,6 +306,24 @@ def test_keywords_args_api(self):
         self.assertEqual(self.db.guess_extension(
             type='image/jpg', strict=False), '.jpg')
 
+    def test_added_types_are_used(self):
+        mimetypes.add_type('testing/default-type', '')
+        mime_type, _ = mimetypes.guess_type('')
+        self.assertEqual(mime_type, 'testing/default-type')
+
+        mime_type, _ = mimetypes.guess_type('test.myext')
+        self.assertEqual(mime_type, None)
+
+        mimetypes.add_type('testing/type', '.myext')
+        mime_type, _ = mimetypes.guess_type('test.myext')
+        self.assertEqual(mime_type, 'testing/type')
+
+    def test_add_type_with_undotted_extension_raises_exception(self):
+        with self.assertRaisesRegex(
+            ValueError, "Extensions should start with a '.' or be empty"
+        ):
+            mimetypes.add_type('testing/type', 'undotted')
+
 
 @unittest.skipUnless(sys.platform.startswith("win"), "Windows only")
 class Win32MimeTypesTestCase(unittest.TestCase):
diff --git a/Misc/NEWS.d/next/Library/2019-09-10-09-28-52.gh-issue-75223.VyAJS9.rst b/Misc/NEWS.d/next/Library/2019-09-10-09-28-52.gh-issue-75223.VyAJS9.rst
new file mode 100644
index 00000000000000..bafcbe1b714b72
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2019-09-10-09-28-52.gh-issue-75223.VyAJS9.rst
@@ -0,0 +1 @@
+Reject undotted extensions in :meth:`mimetypes.MimeTypes.add_type`.