diff --git a/Lib/argparse.py b/Lib/argparse.py
index 2144c81886ad19..3b0fbc5354f281 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -1316,7 +1316,10 @@ def __call__(self, parser, namespace, values, option_string=None):
         # namespace for the relevant parts.
         subnamespace, arg_strings = subparser.parse_known_args(arg_strings, None)
         for key, value in vars(subnamespace).items():
-            setattr(namespace, key, value)
+            if key != '__defaults__':
+                setattr(namespace, key, value)
+        if hasattr(namespace, '__defaults__'):
+            namespace.__defaults__.update(subnamespace.__defaults__)
 
         if arg_strings:
             if not hasattr(namespace, _UNRECOGNIZED_ARGS_ATTR):
@@ -1398,6 +1401,9 @@ def __repr__(self):
 class Namespace(_AttributeHolder):
     """Simple object for storing attributes.
 
+    Default values are stored in a dict named `__defaults__` so they won't
+    override the given values.
+
     Implements equality by attribute names and values, and provides a simple
     string representation.
     """
@@ -1405,16 +1411,34 @@ class Namespace(_AttributeHolder):
     def __init__(self, **kwargs):
         for name in kwargs:
             setattr(self, name, kwargs[name])
+        self.__defaults__ = {}
+
+    def _get_kwargs(self):
+        kwargs = self.__defaults__ | self.__dict__
+        kwargs.pop('__defaults__', None)
+        return list(kwargs.items())
+
+    def __getattr__(self, name):
+        try:
+            return self.__defaults__[name]
+        except KeyError:
+            raise AttributeError(name)
 
     def __eq__(self, other):
         if not isinstance(other, Namespace):
             return NotImplemented
-        return vars(self) == vars(other)
+        return dict(self._get_kwargs()) == dict(other._get_kwargs())
 
     def __contains__(self, key):
-        return key in self.__dict__
+        return key in self.__dict__ or key in self.__defaults__
 
 
+def _set_default(namespace, dest, value):
+    if not hasattr(namespace, '__defaults__'):
+        setattr(namespace, dest, value)
+    else:
+        namespace.__defaults__[dest] = value
+
 class _ActionsContainer(object):
 
     def __init__(self,
@@ -2025,14 +2049,13 @@ def _parse_known_args2(self, args, namespace, intermixed):
         # add any action defaults that aren't present
         for action in self._actions:
             if action.dest is not SUPPRESS:
-                if not hasattr(namespace, action.dest):
-                    if action.default is not SUPPRESS:
-                        setattr(namespace, action.dest, action.default)
+                if action.default is not SUPPRESS and not hasattr(namespace, action.dest):
+                    _set_default(namespace, action.dest, action.default)
 
         # add any parser defaults that aren't present
         for dest in self._defaults:
             if not hasattr(namespace, dest):
-                setattr(namespace, dest, self._defaults[dest])
+                _set_default(namespace, dest, self._defaults[dest])
 
         # parse the arguments and exit if there are any errors
         if self.exit_on_error:
@@ -2330,7 +2353,7 @@ def consume_positionals(start_index):
                         isinstance(action.default, str) and
                         hasattr(namespace, action.dest) and
                         action.default is getattr(namespace, action.dest)):
-                        setattr(namespace, action.dest,
+                        _set_default(namespace, action.dest,
                                 self._get_value(action, action.default))
 
         if required_actions:
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index fc73174d98cd6f..9c5756dca51304 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -121,18 +121,7 @@ def __init__(self, *args, **kwargs):
         self.kwargs = kwargs
 
 
-class NS(object):
-
-    def __init__(self, **kwargs):
-        self.__dict__.update(kwargs)
-
-    def __repr__(self):
-        sorted_items = sorted(self.__dict__.items())
-        kwarg_str = ', '.join(['%s=%r' % tup for tup in sorted_items])
-        return '%s(%s)' % (type(self).__name__, kwarg_str)
-
-    def __eq__(self, other):
-        return vars(self) == vars(other)
+NS = argparse.Namespace
 
 
 class ArgumentParserError(Exception):
@@ -2474,6 +2463,16 @@ def _get_parser(self, subparser_help=False, prefix_chars=None,
         # return the main parser
         return parser
 
+    def _get_parser_with_shared_option(self):
+        parser = ErrorRaisingArgumentParser(prog='PROG', description='main description')
+        parser.add_argument('-f', '--foo', default='0')
+        subparsers = parser.add_subparsers()
+        parser1 = subparsers.add_parser('1')
+        parser1.add_argument('-f', '--foo', default='1')
+        parser2 = subparsers.add_parser('2')
+        parser2.add_argument('-f', '--foo', default='2')
+        return parser
+
     def setUp(self):
         super().setUp()
         self.parser = self._get_parser()
@@ -2940,6 +2939,14 @@ def test_alias_help(self):
                 3                   3 help
             """))
 
+    def test_subparsers_with_shared_option(self):
+        parser = self._get_parser_with_shared_option()
+        self.assertEqual(parser.parse_args([]), NS(foo='0'))
+        self.assertEqual(parser.parse_args(['1']), NS(foo='1'))
+        self.assertEqual(parser.parse_args(['2']), NS(foo='2'))
+        self.assertEqual(parser.parse_args(['-f', '10', '1', '-f', '42']), NS(foo='42'))
+        self.assertEqual(parser.parse_args(['1'], NS(foo='42')), NS(foo='42'))
+
 # ============
 # Groups tests
 # ============
diff --git a/Misc/NEWS.d/next/Library/2021-12-21-08-34-36.bpo-45235.OV9_9i.rst b/Misc/NEWS.d/next/Library/2021-12-21-08-34-36.bpo-45235.OV9_9i.rst
new file mode 100644
index 00000000000000..06c70cb3480948
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2021-12-21-08-34-36.bpo-45235.OV9_9i.rst
@@ -0,0 +1 @@
+Fix an issue that subparsers defaults override the existing values in the argparse Namespace.