diff --git a/Lib/argparse.py b/Lib/argparse.py
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -2042,6 +2042,18 @@
         # return the list of arg string counts
         return result
 
+    def _get_optional_action(self, arg_string):
+        if arg_string in self._option_string_actions:
+            return self._option_string_actions[arg_string]
+        if self._subparsers is not None:
+            for action in self._subparsers._actions:
+                if isinstance(action, _SubParsersAction):
+                    for parser in action._name_parser_map.values():
+                        sub_action = parser._get_optional_action(arg_string)
+                        if sub_action is not None:
+                            return sub_action
+        return None
+
     def _parse_optional(self, arg_string):
         # if it's an empty string, it was meant to be a positional
         if not arg_string:
@@ -2052,8 +2064,8 @@
             return None
 
         # if the option string is present in the parser, return the action
-        if arg_string in self._option_string_actions:
-            action = self._option_string_actions[arg_string]
+        action = self._get_optional_action(arg_string)
+        if action is not None:
             return action, arg_string, None
 
         # if it's just a single character, it was meant to be positional