diff --git a/Lib/test/test_hashlib.py b/Lib/test/test_hashlib.py
index b511c160c9260f..a718be415b9905 100644
--- a/Lib/test/test_hashlib.py
+++ b/Lib/test/test_hashlib.py
@@ -1505,6 +1505,12 @@ def test_HACL_sha3_attributes(self, size, attrname):
     def test_HACL_shake_attributes(self, size, attrname):
         self.check_HACL_attribute(_sha3, f"shake_{size}", attrname)
 
+    @requires_blake2
+    @support.subTests("version", ["blake2s", "blake2b"])
+    @support.subTests("attrname", ["block_size", "digest_size"])
+    def test_HACL_blake2_attributes(self, version, attrname):
+        self.check_HACL_attribute(_blake2, version, attrname)
+
 
 if __name__ == "__main__":
     unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2026-08-15-13-18-01.gh-issue-155835.mlyLWp.rst b/Misc/NEWS.d/next/Library/2026-08-15-13-18-01.gh-issue-155835.mlyLWp.rst
new file mode 100644
index 00000000000000..9ef7d0694e66c3
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2026-08-15-13-18-01.gh-issue-155835.mlyLWp.rst
@@ -0,0 +1,2 @@
+:mod:`hashlib`: Fix a data race when accessing :attr:`~hashlib.hash.digest_size`
+on BLAKE-2 objects. Patch by Bénédikt Tran.
diff --git a/Modules/blake2module.c b/Modules/blake2module.c
index ac7265bb9d6836..c69c4259b12666 100644
--- a/Modules/blake2module.c
+++ b/Modules/blake2module.c
@@ -950,7 +950,9 @@ static PyObject *
 py_blake2b_get_digest_size(PyObject *op, void *Py_UNUSED(closure))
 {
     Blake2Object *self = _Blake2Object_CAST(op);
+    HASHLIB_ACQUIRE_LOCK(self);
     Hacl_Hash_Blake2b_index info = hacl_get_blake2_info(self);
+    HASHLIB_RELEASE_LOCK(self);
     return PyLong_FromLong(info.digest_length);
 }