diff --git a/Lib/test/test_free_threading/test_set.py b/Lib/test/test_free_threading/test_set.py
index 9dd3d68d5dad13..1d5e09cb4089d4 100644
--- a/Lib/test/test_free_threading/test_set.py
+++ b/Lib/test/test_free_threading/test_set.py
@@ -148,6 +148,32 @@ def read_set():
         for t in threads:
             t.join()
 
+    def test_iter_length_hint_mutate(self):
+        s = set(range(2000))
+        it = iter(s)
+        stop = Event()
+
+        def reader():
+            while not stop.is_set():
+                it.__length_hint__()
+
+        def writer():
+            i = 0
+            while not stop.is_set():
+                s.add(i)
+                s.discard(i - 1)
+                i += 1
+
+        threads = [Thread(target=reader) for _ in range(4)]
+        threads.append(Thread(target=writer))
+
+        for t in threads:
+            t.start()
+
+        stop.set()
+
+        for t in threads:
+            t.join()
 
 @threading_helper.requires_working_threading()
 class SmallSetTest(RaceTestBase, unittest.TestCase):
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-01-31-03-40-28.gh-issue-144356.otfq_X.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-01-31-03-40-28.gh-issue-144356.otfq_X.rst
new file mode 100644
index 00000000000000..d5d67ad1e8dbb3
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-01-31-03-40-28.gh-issue-144356.otfq_X.rst
@@ -0,0 +1 @@
+Fix a data race in ``set_iterator.__length_hint__`` under ``Py_GIL_DISABLED``.
diff --git a/Objects/setobject.c b/Objects/setobject.c
index 5d4d1812282eed..85001c980b6f29 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -1056,7 +1056,8 @@ setiter_len(PyObject *op, PyObject *Py_UNUSED(ignored))
 {
     setiterobject *si = (setiterobject*)op;
     Py_ssize_t len = 0;
-    if (si->si_set != NULL && si->si_used == si->si_set->used)
+    PySetObject *so = si->si_set;
+    if (so != NULL && si->si_used == FT_ATOMIC_LOAD_SSIZE_RELAXED(so->used))
         len = si->len;
     return PyLong_FromSsize_t(len);
 }