diff --git a/Lib/argparse.py b/Lib/argparse.py
index 5ad7e13a480..83878b15274 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -1198,9 +1198,13 @@ def __init__(self, **kwargs):
             setattr(self, name, kwargs[name])
 
     def __eq__(self, other):
+        if not isinstance(other, Namespace):
+            return NotImplemented
         return vars(self) == vars(other)
 
     def __ne__(self, other):
+        if not isinstance(other, Namespace):
+            return NotImplemented
         return not (self == other)
 
     def __contains__(self, key):
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index 5c77ec89d0b..8a4fdb90a6d 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -4551,6 +4551,12 @@ def test_equality(self):
         self.assertTrue(ns2 != ns3)
         self.assertTrue(ns2 != ns4)
 
+    def test_equality_returns_notimplemeted(self):
+        # See issue 21481
+        ns = argparse.Namespace(a=1, b=2)
+        self.assertIs(ns.__eq__(None), NotImplemented)
+        self.assertIs(ns.__ne__(None), NotImplemented)
+
 
 # ===================
 # File encoding tests
diff --git a/Misc/NEWS b/Misc/NEWS
index cdce6f06ffa..13ff667f1b7 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -24,6 +24,9 @@ Library
 - Issue #14710: pkgutil.find_loader() no longer raises an exception when a
   module doesn't exist.
 
+- Issue #21481:  Argparse equality and inequality tests now return 
+  NotImplemented when comparing to an unknown type.
+
 - Issue #8743: Fix interoperability between set objects and the
   collections.Set() abstract base class.