diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-06-03-16-27-00.gh-issue-150858.j2dSkD.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-06-03-16-27-00.gh-issue-150858.j2dSkD.rst
new file mode 100644
index 00000000000000..f49cff2c3477f5
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2026-06-03-16-27-00.gh-issue-150858.j2dSkD.rst
@@ -0,0 +1,2 @@
+Fix a data race in :c:func:`type_set_qualname` where concurrent calls
+``__qualname__`` could cause a memory corruption on free-threaded builds.
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index e0464fe6475cfd..de5bb5f1f2a0df 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -1594,7 +1594,12 @@ type_set_qualname(PyObject *tp, PyObject *value, void *context)
     }
 
     et = (PyHeapTypeObject*)type;
-    Py_SETREF(et->ht_qualname, Py_NewRef(value));
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    _PyEval_StopTheWorld(interp);
+    PyObject *old_qualname = et->ht_qualname;
+    et->ht_qualname = Py_NewRef(value);
+    _PyEval_StartTheWorld(interp);
+    Py_DECREF(old_qualname);
     return 0;
 }