diff --git a/Lib/ipaddress.py b/Lib/ipaddress.py
index cb41c522883..703fa289dda 100644
--- a/Lib/ipaddress.py
+++ b/Lib/ipaddress.py
@@ -2375,6 +2375,8 @@ class _IPv6Constants:
         IPv6Network('2001:db8::/32'),
         # IANA says N/A, let's consider it not globally reachable to be safe
         IPv6Network('2002::/16'),
+        # RFC 9637: https://www.rfc-editor.org/rfc/rfc9637.html#section-6-2.2
+        IPv6Network('3fff::/20'),
         IPv6Network('fc00::/7'),
         IPv6Network('fe80::/10'),
         ]
diff --git a/Lib/test/test_ipaddress.py b/Lib/test/test_ipaddress.py
index aad779dd9b2..b1ac2b94f41 100644
--- a/Lib/test/test_ipaddress.py
+++ b/Lib/test/test_ipaddress.py
@@ -2433,6 +2433,8 @@ class IpaddrUnitTest(unittest.TestCase):
         self.assertTrue(ipaddress.ip_address('2001:30::').is_global)
         self.assertFalse(ipaddress.ip_address('2001:40::').is_global)
         self.assertFalse(ipaddress.ip_address('2002::').is_global)
+        # gh-124217: conform with RFC 9637
+        self.assertFalse(ipaddress.ip_address('3fff::').is_global)
 
         # some generic IETF reserved addresses
         self.assertEqual(True, ipaddress.ip_address('100::').is_reserved)
diff --git a/Misc/NEWS.d/next/Library/2024-09-19-20-15-00.gh-issue-124217.j0KlQB.rst b/Misc/NEWS.d/next/Library/2024-09-19-20-15-00.gh-issue-124217.j0KlQB.rst
new file mode 100644
index 00000000000..46f9866f8d4
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2024-09-19-20-15-00.gh-issue-124217.j0KlQB.rst
@@ -0,0 +1 @@
+Add RFC 9637 reserved IPv6 block ``3fff::/20`` in :mod:`ipaddress` module.