diff --git a/src/cachetools/__init__.py b/src/cachetools/__init__.py
index 1b299c5..53739aa 100644
--- a/src/cachetools/__init__.py
+++ b/src/cachetools/__init__.py
@@ -625,10 +625,26 @@ def __getitem__(self, key, cache_getitem=Cache.__getitem__):
         else:
             return cache_getitem(self, key)
 
-    def __setitem__(self, key, value, cache_setitem=Cache.__setitem__):
+    def __setitem__(
+        self,
+        key,
+        value,
+        cache_setitem=Cache.__setitem__,
+        cache_delitem=Cache.__delitem__,
+    ):
         with self.timer as time:
             expires = self.__ttu(key, value, time)
             if not (time < expires):
+                # The new value is already expired, so it is not stored.
+                # If the key still holds a previous (possibly still valid)
+                # value, drop it as well -- otherwise the assignment would
+                # be silently ignored and the stale value would persist.
+                try:
+                    self.__items.pop(key).removed = True
+                except KeyError:
+                    pass
+                else:
+                    cache_delitem(self, key)
                 return  # skip expired items
             self.expire(time)
             cache_setitem(self, key, value)
diff --git a/tests/test_tlru.py b/tests/test_tlru.py
index b1263d7..b0dac65 100644
--- a/tests/test_tlru.py
+++ b/tests/test_tlru.py
@@ -144,6 +144,26 @@ def ttu(_k, _v, t):
         self.assertEqual(cache[4], 4)
         self.assertEqual(cache[5], 5)
 
+    def test_overwrite_existing_with_expired(self):
+        # Overwriting an existing key with an already-expired value must not
+        # silently retain the previous value: the assignment cannot store the
+        # new (dead-on-arrival) value, so the key has to be evicted instead.
+        def ttu(_k, value, t):
+            return t + value
+
+        cache = TLRUCache[int, int, int](maxsize=2, ttu=ttu, timer=Timer())
+        cache[1] = 5
+        self.assertEqual(cache[1], 5)
+        self.assertEqual(len(cache), 1)
+        self.assertEqual(cache.currsize, 1)
+
+        cache[1] = 0  # ttu -> t, i.e. not (t < t): immediately expired
+        self.assertNotIn(1, cache)
+        self.assertIsNone(cache.get(1))
+        self.assertEqual(len(cache), 0)
+        self.assertEqual(cache.currsize, 0)
+        self.assertEqual(set(cache), set())
+
     def test_ttu_expire(self):
         def ttu(_k, _v, t):
             return t + 3