diff --git a/Lib/argparse.py b/Lib/argparse.py
index 83878b15274..4d26b0701bb 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -1122,7 +1122,14 @@ class _SubParsersAction(Action):
         # parse all the remaining options into the namespace
         # store any unrecognized options on the object, so that the top
         # level parser can decide what to do with them
-        namespace, arg_strings = parser.parse_known_args(arg_strings, namespace)
+
+        # In case this subparser defines new defaults, we parse them
+        # in a new namespace object and then update the original
+        # namespace for the relevant parts.
+        subnamespace, arg_strings = parser.parse_known_args(arg_strings, None)
+        for key, value in vars(subnamespace).items():
+            setattr(namespace, key, value)
+
         if arg_strings:
             vars(namespace).setdefault(_UNRECOGNIZED_ARGS_ATTR, [])
             getattr(namespace, _UNRECOGNIZED_ARGS_ATTR).extend(arg_strings)
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index b61572270e9..98ccfe44843 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -2795,6 +2795,13 @@ class TestSetDefaults(TestCase):
         parser = ErrorRaisingArgumentParser(parents=[parent])
         self.assertEqual(NS(x='foo'), parser.parse_args([]))
 
+    def test_set_defaults_on_parent_and_subparser(self):
+        parser = argparse.ArgumentParser()
+        xparser = parser.add_subparsers().add_parser('X')
+        parser.set_defaults(foo=1)
+        xparser.set_defaults(foo=2)
+        self.assertEqual(NS(foo=2), parser.parse_args(['X']))
+
     def test_set_defaults_same_as_add_argument(self):
         parser = ErrorRaisingArgumentParser()
         parser.set_defaults(w='W', x='X', y='Y', z='Z')
diff --git a/Misc/NEWS b/Misc/NEWS
index e04f4cc02d3..9c50f3e22ab 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -33,6 +33,9 @@ Core and Builtins
 Library
 -------
 
+- Issue #9351: Defaults set with set_defaults on an argparse subparser
+  are no longer ignored when also set on the parent parser.
+
 - Issue #21991: Make email.headerregistry's header 'params' attributes
   be read-only (MappingProxyType).  Previously the dictionary was modifiable
   but a new one was created on each access of the attribute.