diff --git a/Lib/test/test_hashlib.py b/Lib/test/test_hashlib.py
index 253a8f455853f58..ddfacdd14e0a533 100644
--- a/Lib/test/test_hashlib.py
+++ b/Lib/test/test_hashlib.py
@@ -18,6 +18,8 @@
 import tempfile
 import threading
 import unittest
+from functools import partial
+from operator import attrgetter
 from test import support
 from test.support import _4G, bigmemtest
 from test.support import hashlib_helper
@@ -56,14 +58,12 @@ def get_fips_mode():
     import _blake2
 except ImportError:
     _blake2 = None
-
 requires_blake2 = unittest.skipUnless(_blake2, 'requires _blake2')
 
 try:
     import _sha3
 except ImportError:
     _sha3 = None
-
 requires_sha3 = unittest.skipUnless(_sha3, 'requires _sha3')
 
 
@@ -1418,5 +1418,42 @@ def scrypt(password=b"password", /, **kwargs):
         self.assertRaises(numeric_exc_types, scrypt, dklen=MAX_DKLEN + 1)
 
 
+@threading_helper.requires_working_threading()
+class TestTSAN(unittest.TestCase):
+
+    @threading_helper.reap_threads
+    def check_attribute(self, write, read, expected, nthreads=8):
+        ready = threading.Event()
+        barrier = threading.Barrier(nthreads)
+
+        def writer():
+            barrier.wait()
+            while not ready.is_set():
+                write()
+
+        def reader():
+            barrier.wait()
+            while not ready.is_set():
+                self.assertEqual(read(), expected)
+
+        targets = [writer if i % 2 else reader for i in range(nthreads)]
+        workers = [threading.Thread(target=target) for target in targets]
+        with threading_helper.start_threads(workers, unlock=ready.set):
+            pass
+
+    def check_HACL_attribute(self, module, version, attrname):
+        blob = b"A" * 65536
+        obj = getattr(module, version)()
+        update = partial(obj.update, blob)
+        read = attrgetter(attrname)
+        self.check_attribute(update, partial(read, obj), read(obj))
+
+    @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 000000000000000..9ef7d0694e66c31
--- /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 ac7265bb9d6836c..c69c4259b12666f 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);
 }