diff --git a/Lib/argparse.py b/Lib/argparse.py
index e4892955e4ff09..cb157da87bf37e 100644
--- a/Lib/argparse.py
+++ b/Lib/argparse.py
@@ -1405,7 +1405,7 @@ def __init__(self,
         self._defaults = {}
 
         # determines whether an "option" looks like a negative number
-        self._negative_number_matcher = _re.compile(r'^-\d+$|^-\d*\.\d+$')
+        self._negative_number_matcher = _re.compile(r'^-\d[\d_]*(\.\d[\d_]*)?$')
 
         # whether or not there are any optionals that look like negative
         # numbers -- uses a list so it can be shared and edited
diff --git a/Lib/test/test_argparse.py b/Lib/test/test_argparse.py
index 6a7723e8cd1e26..4810331ef6ab7e 100644
--- a/Lib/test/test_argparse.py
+++ b/Lib/test/test_argparse.py
@@ -2092,6 +2092,47 @@ class TestActionExtend(ParserTestCase):
         ('--foo f1 --foo f2 f3 f4', NS(foo=['f1', 'f2', 'f3', 'f4'])),
     ]
 
+
+class TestNegativeNumber(ParserTestCase):
+    """Test parsing negative numbers"""
+
+    argument_signatures = [
+        Sig('--int', type=int),
+        Sig('--float', type=float),
+    ]
+    failures = [
+        '--float -_.45',
+        '--float -1__000.0',
+        '--int -1__000',
+    ]
+    successes = [
+        ('--int -1000 --float -1000.0', NS(int=-1000, float=-1000.0)),
+        ('--int -1_000 --float -1_000.0', NS(int=-1000, float=-1000.0)),
+        ('--int -1_000_000 --float -1_000_000.0', NS(int=-1000000, float=-1000000.0)),
+        ('--float -1_000.0', NS(int=None, float=-1000.0)),
+        ('--float -1_000_000.0_0', NS(int=None, float=-1000000.0)),
+    ]
+
+class TestInvalidAction(TestCase):
+    """Test invalid user defined Action"""
+
+    class ActionWithoutCall(argparse.Action):
+        pass
+
+    def test_invalid_type(self):
+        parser = argparse.ArgumentParser()
+
+        parser.add_argument('--foo', action=self.ActionWithoutCall)
+        self.assertRaises(NotImplementedError, parser.parse_args, ['--foo', 'bar'])
+
+    def test_modified_invalid_action(self):
+        parser = ErrorRaisingArgumentParser()
+        action = parser.add_argument('--foo')
+        # Someone got crazy and did this
+        action.type = 1
+        self.assertRaises(ArgumentParserError, parser.parse_args, ['--foo', 'bar'])
+
+
 # ================
 # Subparsers tests
 # ================
diff --git a/Misc/NEWS.d/next/Library/2024-09-11-19-05-32.gh-issue-123945.jLwybB.rst b/Misc/NEWS.d/next/Library/2024-09-11-19-05-32.gh-issue-123945.jLwybB.rst
new file mode 100644
index 00000000000000..26b0ac80b1b3fd
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-09-11-19-05-32.gh-issue-123945.jLwybB.rst
@@ -0,0 +1 @@
+Fix a bug where :mod:`argparse` doesn't recognize negative numbers with underscores