diff --git a/Lib/hmac.py b/Lib/hmac.py
index 16022c9ceb5439..2d6016cda11c0e 100644
--- a/Lib/hmac.py
+++ b/Lib/hmac.py
@@ -159,6 +159,7 @@ def copy(self):
         # Call __new__ directly to avoid the expensive __init__.
         other = self.__class__.__new__(self.__class__)
         other.digest_size = self.digest_size
+        other.block_size = self.block_size
         if self._hmac:
             other._hmac = self._hmac.copy()
             other._inner = other._outer = None
diff --git a/Lib/test/test_hmac.py b/Lib/test/test_hmac.py
index 344c6ddf28afcf..1d270b2a43c184 100644
--- a/Lib/test/test_hmac.py
+++ b/Lib/test/test_hmac.py
@@ -1076,6 +1076,15 @@ def test_properties(self):
         self.assertEqual(h.digest_size, self.digest_size)
         self.assertEqual(h.block_size, self.block_size)
 
+    def test_copy(self):
+        # Test a generic copy() and the attributes it exposes.
+        # See https://github.com/python/cpython/issues/142451.
+        h1 = self.hmac_new(b"my secret key", digestmod=self.digestname)
+        h2 = h1.copy()
+        self.assertEqual(h1.name, h2.name)
+        self.assertEqual(h1.digest_size, h2.digest_size)
+        self.assertEqual(h1.block_size, h2.block_size)
+
     def test_repr(self):
         # HMAC object representation may differ across implementations
         raise NotImplementedError
diff --git a/Misc/NEWS.d/next/Library/2025-12-14-10-00-23.gh-issue-142451._rkf2S.rst b/Misc/NEWS.d/next/Library/2025-12-14-10-00-23.gh-issue-142451._rkf2S.rst
new file mode 100644
index 00000000000000..cceb572f503eda
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-12-14-10-00-23.gh-issue-142451._rkf2S.rst
@@ -0,0 +1,3 @@
+:mod:`hmac`: Ensure that the :attr:`HMAC.block_size <hmac.HMAC.block_size>`
+attribute is correctly copied by :meth:`HMAC.copy <hmac.HMAC.copy>`. Patch
+by Bénédikt Tran.