diff --git a/src/cachelib/mongodb.py b/src/cachelib/mongodb.py
index a7b19f7..d56bf20 100644
--- a/src/cachelib/mongodb.py
+++ b/src/cachelib/mongodb.py
@@ -49,7 +49,7 @@ def __init__(
         if "id" not in all_keys:
             self.client.create_index("id", unique=True)
         if "expiration" not in all_keys:
-            self.client.create_index("expiration")
+            self.client.create_index("expiration", expireAfterSeconds=0)
         self.key_prefix = key_prefix or ""
         self.collection = collection
 
diff --git a/tests/test_mongodb_cache.py b/tests/test_mongodb_cache.py
index 20ce063..85afd95 100644
--- a/tests/test_mongodb_cache.py
+++ b/tests/test_mongodb_cache.py
@@ -1,3 +1,5 @@
+import time
+
 import pytest
 from clear import ClearTests
 from common import CommonTests
@@ -28,4 +30,17 @@ def _factory(self, *args, **kwargs):
 
 
 class TestMongoDbCache(CommonTests, ClearTests, HasTests):
-    pass
+    def test_auto_expire(self):
+        """Test that MongoDB's TTL index automatically expires cache entries."""
+        cache = self.cache_factory()
+        # Set a cache entry with a short timeout
+        cache.set("auto_expire_key", "value", timeout=2)
+
+        # Verify it exists
+        assert cache.get("auto_expire_key") == "value"
+
+        # Wait for expiration (add buffer time for MongoDB TTL monitor)
+        time.sleep(5)
+
+        # Verify it has expired
+        assert cache.get("auto_expire_key") is None