diff --git a/CHANGES/1327.bugfix.rst b/CHANGES/1327.bugfix.rst
new file mode 100644
index 000000000..6fa502c68
--- /dev/null
+++ b/CHANGES/1327.bugfix.rst
@@ -0,0 +1,3 @@
+Fixed insertion race on free-threaded mode.
+
+-- by :user:`Vizonex`
\ No newline at end of file
diff --git a/multidict/_multilib/hashtable.h b/multidict/_multilib/hashtable.h
index f2ba9868a..ff6a917f6 100644
--- a/multidict/_multilib/hashtable.h
+++ b/multidict/_multilib/hashtable.h
@@ -412,6 +412,7 @@ _md_add_with_hash_steal_refs(MultiDictObject *md, Py_hash_t hash,
                              PyObject *identity, PyObject *key,
                              PyObject *value)
 {
+    Py_BEGIN_CRITICAL_SECTION(md);
     htkeys_t *keys = md->keys;
     if (keys->usable <= 0 || keys == &empty_htkeys) {
         /* Need to resize. */
@@ -435,6 +436,7 @@ _md_add_with_hash_steal_refs(MultiDictObject *md, Py_hash_t hash,
     md->used += 1;
     keys->usable -= 1;
     keys->nentries += 1;
+    Py_END_CRITICAL_SECTION();
     return 0;
 }
 
diff --git a/tests/isolated/multidict_add_ft.py b/tests/isolated/multidict_add_ft.py
new file mode 100644
index 000000000..f31ad1e35
--- /dev/null
+++ b/tests/isolated/multidict_add_ft.py
@@ -0,0 +1,30 @@
+import os
+import subprocess
+import sys
+import sysconfig
+import textwrap
+
+FREETHREADED = bool(sysconfig.get_config_var("Py_GIL_DISABLED"))
+
+if __name__ == "__main__" and FREETHREADED:
+    child = textwrap.dedent("""
+        import threading
+        from multidict import MultiDict
+        md: MultiDict[int] = MultiDict()
+        def worker(tid: int):
+            for i in range(800):
+                md.add(f"k{tid}_{i}", i)
+        ts = [threading.Thread(target=worker, args=(tid,), daemon=True) for tid in range(4)]
+        for t in ts: 
+            t.start()
+        for t in ts: 
+            t.join()
+        print(f"missing: {4*800 - len(md)}")
+    """)
+    subprocess.run(
+        [sys.executable, "-c", child],
+        env={**os.environ, "PYTHON_GIL": "0"},
+        capture_output=True,
+        timeout=60,
+        check=True,
+    )
diff --git a/tests/test_leaks.py b/tests/test_leaks.py
index c39abe245..f7d4e86af 100644
--- a/tests/test_leaks.py
+++ b/tests/test_leaks.py
@@ -11,6 +11,7 @@
 @pytest.mark.parametrize(
     ("script"),
     (
+        "multidict_add_ft.py",
         "multidict_extend_dict.py",
         "multidict_extend_multidict.py",
         "multidict_extend_tuple.py",