diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2026-10-21-21-26-10.gh-issue-159739.Aw61LR.rst b/Misc/NEWS.d/next/Core_and_Builtins/2026-10-21-21-26-10.gh-issue-159739.Aw61LR.rst
@@ -0,0 +1 @@
+Fix a race in ``list.__sizeof__()`` when a list is concurrently resized on a free-threaded build.
diff --git a/Objects/listobject.c b/Objects/listobject.c
@@ -137,7 +137,7 @@
 py_list_resize(PyListObject *self, Py_ssize_t newsize)
 {
    size_t new_allocated, target_bytes;
    Py_ssize_t allocated = self->allocated;

    /* This over-allocates proportional to the list size, making room
     * for additional growth.  The over-allocation is mild, but is
@@ -3560,10 +3560,12 @@
 {
     size_t res = _PyObject_SIZE(Py_TYPE(self));
 #ifdef Py_GIL_DISABLED
-    PyObject **ob_item = _Py_atomic_load_ptr(&self->ob_item);
+    Py_BEGIN_CRITICAL_SECTION(self);
+    PyObject **ob_item = self->ob_item;
     if (ob_item != NULL) {
         res += list_capacity(ob_item) * sizeof(PyObject *);
     }
+    Py_END_CRITICAL_SECTION();
 #else
     res += (size_t)self->allocated * sizeof(PyObject *);
 #endif
Original file line number	Diff line number	Diff line change
		@@ -0,0 +1 @@
		Fix a race in ``list.__sizeof__()`` when a list is concurrently resized on a free-threaded build.