diff --git a/Lib/test/test_dict.py b/Lib/test/test_dict.py
index 60c62430370e96..2e6c2bbdf19409 100644
--- a/Lib/test/test_dict.py
+++ b/Lib/test/test_dict.py
@@ -1601,6 +1601,26 @@ def __hash__(self):
         with self.assertRaises(KeyError):
             d.get(key2)
 
+    def test_clear_at_lookup(self):
+        class X:
+            def __hash__(self):
+                return 1
+            def __eq__(self, other):
+                nonlocal d
+                d.clear()
+
+        d = {}
+        for _ in range(10):
+            d[X()] = None
+
+        self.assertEqual(len(d), 1)
+
+        d = {}
+        for _ in range(10):
+            d.setdefault(X(), None)
+
+        self.assertEqual(len(d), 1)
+
 
 class CAPITest(unittest.TestCase):
 
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2025-10-24-20-42-33.gh-issue-140551.-9swrl.rst b/Misc/NEWS.d/next/Core_and_Builtins/2025-10-24-20-42-33.gh-issue-140551.-9swrl.rst
new file mode 100644
index 00000000000000..8fd9b46c0aeabe
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2025-10-24-20-42-33.gh-issue-140551.-9swrl.rst
@@ -0,0 +1,2 @@
+Fixed crash in :class:`dict` if :meth:`dict.clear` is called at the lookup
+stage. Patch by Mikhail Efimov and Inada Naoki.
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 73d4db4cac7963..65eed151c2829d 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -1775,6 +1775,14 @@ static inline int
 insert_combined_dict(PyInterpreterState *interp, PyDictObject *mp,
                      Py_hash_t hash, PyObject *key, PyObject *value)
 {
+    // gh-140551: If dict was cleared in _Py_dict_lookup,
+    // we have to resize one more time to force general key kind.
+    if (DK_IS_UNICODE(mp->ma_keys) && !PyUnicode_CheckExact(key)) {
+        if (insertion_resize(mp, 0) < 0)
+            return -1;
+        assert(mp->ma_keys->dk_kind == DICT_KEYS_GENERAL);
+    }
+
     if (mp->ma_keys->dk_usable <= 0) {
         /* Need to resize. */
         if (insertion_resize(mp, 1) < 0) {
@@ -1871,38 +1879,31 @@ insertdict(PyInterpreterState *interp, PyDictObject *mp,
            PyObject *key, Py_hash_t hash, PyObject *value)
 {
     PyObject *old_value;
+    Py_ssize_t ix;
 
     ASSERT_DICT_LOCKED(mp);
 
-    if (DK_IS_UNICODE(mp->ma_keys) && !PyUnicode_CheckExact(key)) {
-        if (insertion_resize(mp, 0) < 0)
-            goto Fail;
-        assert(mp->ma_keys->dk_kind == DICT_KEYS_GENERAL);
-    }
-
-    if (_PyDict_HasSplitTable(mp)) {
-        Py_ssize_t ix = insert_split_key(mp->ma_keys, key, hash);
+    if (_PyDict_HasSplitTable(mp) && PyUnicode_CheckExact(key)) {
+        ix = insert_split_key(mp->ma_keys, key, hash);
         if (ix != DKIX_EMPTY) {
             insert_split_value(interp, mp, key, value, ix);
             Py_DECREF(key);
             Py_DECREF(value);
             return 0;
         }
-
-        /* No space in shared keys. Resize and continue below. */
-        if (insertion_resize(mp, 1) < 0) {
+        // No space in shared keys. Go to insert_combined_dict() below.
+    }
+    else {
+        ix = _Py_dict_lookup(mp, key, hash, &old_value);
+        if (ix == DKIX_ERROR)
             goto Fail;
-        }
     }
 
-    Py_ssize_t ix = _Py_dict_lookup(mp, key, hash, &old_value);
-    if (ix == DKIX_ERROR)
-        goto Fail;
-
     if (ix == DKIX_EMPTY) {
-        assert(!_PyDict_HasSplitTable(mp));
-        /* Insert into new slot. */
-        assert(old_value == NULL);
+        // insert_combined_dict() will convert from non DICT_KEYS_GENERAL table
+        // into DICT_KEYS_GENERAL table if key is not Unicode.
+        // We don't convert it before _Py_dict_lookup because non-Unicode key
+        // may change generic table into Unicode table.
         if (insert_combined_dict(interp, mp, hash, key, value) < 0) {
             goto Fail;
         }
@@ -4374,6 +4375,7 @@ dict_setdefault_ref_lock_held(PyObject *d, PyObject *key, PyObject *default_valu
     PyDictObject *mp = (PyDictObject *)d;
     PyObject *value;
     Py_hash_t hash;
+    Py_ssize_t ix;
     PyInterpreterState *interp = _PyInterpreterState_GET();
 
     ASSERT_DICT_LOCKED(d);
@@ -4409,17 +4411,8 @@ dict_setdefault_ref_lock_held(PyObject *d, PyObject *key, PyObject *default_valu
         return 0;
     }
 
-    if (!PyUnicode_CheckExact(key) && DK_IS_UNICODE(mp->ma_keys)) {
-        if (insertion_resize(mp, 0) < 0) {
-            if (result) {
-                *result = NULL;
-            }
-            return -1;
-        }
-    }
-
-    if (_PyDict_HasSplitTable(mp)) {
-        Py_ssize_t ix = insert_split_key(mp->ma_keys, key, hash);
+    if (_PyDict_HasSplitTable(mp) && PyUnicode_CheckExact(key)) {
+        ix = insert_split_key(mp->ma_keys, key, hash);
         if (ix != DKIX_EMPTY) {
             PyObject *value = mp->ma_values->values[ix];
             int already_present = value != NULL;
@@ -4432,27 +4425,22 @@ dict_setdefault_ref_lock_held(PyObject *d, PyObject *key, PyObject *default_valu
             }
             return already_present;
         }
-
-        /* No space in shared keys. Resize and continue below. */
-        if (insertion_resize(mp, 1) < 0) {
-            goto error;
-        }
+        // No space in shared keys. Go to insert_combined_dict() below.
     }
-
-    assert(!_PyDict_HasSplitTable(mp));
-
-    Py_ssize_t ix = _Py_dict_lookup(mp, key, hash, &value);
-    if (ix == DKIX_ERROR) {
-        if (result) {
-            *result = NULL;
+    else {
+        ix = _Py_dict_lookup(mp, key, hash, &value);
+        if (ix == DKIX_ERROR) {
+            if (result) {
+                *result = NULL;
+            }
+            return -1;
         }
-        return -1;
     }
 
     if (ix == DKIX_EMPTY) {
-        assert(!_PyDict_HasSplitTable(mp));
         value = default_value;
 
+        // See comment to this function in insertdict.
         if (insert_combined_dict(interp, mp, hash, Py_NewRef(key), Py_NewRef(value)) < 0) {
             Py_DECREF(key);
             Py_DECREF(value);
@@ -4477,12 +4465,6 @@ dict_setdefault_ref_lock_held(PyObject *d, PyObject *key, PyObject *default_valu
         *result = incref_result ? Py_NewRef(value) : value;
     }
     return 1;
-
-error:
-    if (result) {
-        *result = NULL;
-    }
-    return -1;
 }
 
 int