diff --git a/Lib/argparse.py b/Lib/argparse.py
index d1a6350c3fda6d..83258cf3e0f37d 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -1534,7 +1534,7 @@ def add_argument(self, *args, **kwargs):
         action_name = kwargs.get('action')
         action_class = self._pop_action_class(kwargs)
         if not callable(action_class):
-            raise ValueError('unknown action {action_class!r}')
+            raise ValueError(f'unknown action {action_class!r}')
         action = action_class(**kwargs)
 
         # raise an error if action for positional argument does not
diff --git a/Misc/NEWS.d/next/Library/2025-05-31-12-08-12.gh-issue-134970.lgSaxq.rst b/Misc/NEWS.d/next/Library/2025-05-31-12-08-12.gh-issue-134970.lgSaxq.rst
new file mode 100644
index 00000000000000..20f53569ef4566
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-05-31-12-08-12.gh-issue-134970.lgSaxq.rst
@@ -0,0 +1,3 @@
+Fix the "unknown action" exception in
+:meth:`argparse.ArgumentParser.add_argument_group` to correctly replace the
+action class.