diff --git a/litellm/types/router.py b/litellm/types/router.py
index d8b2e457706..5c693354629 100644
--- a/litellm/types/router.py
+++ b/litellm/types/router.py
@@ -172,6 +172,11 @@ class TagRateLimitEntry(BaseModel):
     def _validate_key_ttl_seconds(self) -> "TagRateLimitEntry":
         if self.key_ttl_seconds is not None and self.key_ttl_seconds <= 0:
             raise ValueError("key_ttl_seconds must be a positive integer when set")
+        if self.key_ttl_seconds is not None and self.key_ttl_seconds < self.period_seconds:
+            raise ValueError(
+                "key_ttl_seconds must be at least period_seconds when set -- a shorter TTL expires the "
+                "counter before its window rolls over, letting tagged traffic reset to zero and exceed the limit"
+            )
         return self
 
     @model_validator(mode="after")
diff --git a/tests/test_litellm/proxy/hooks/test_tag_rate_limiter.py b/tests/test_litellm/proxy/hooks/test_tag_rate_limiter.py
index 6617e0f9c45..dd547d88a4b 100644
--- a/tests/test_litellm/proxy/hooks/test_tag_rate_limiter.py
+++ b/tests/test_litellm/proxy/hooks/test_tag_rate_limiter.py
@@ -2461,7 +2461,7 @@ def test_ttl_for_concurrency_never_drops_below_the_safety_floor_even_with_a_lowe
     """
     below_floor: Final = 10
     assert (
-        _PROXY_TagRateLimiter._ttl_for(_concurrency_limit(period_seconds=60, key_ttl_seconds=below_floor))
+        _PROXY_TagRateLimiter._ttl_for(_concurrency_limit(period_seconds=5, key_ttl_seconds=below_floor))
         == _CONCURRENCY_MIN_SAFETY_TTL_SECONDS
     )
 
@@ -2471,6 +2471,17 @@ def test_tag_rate_limit_entry_rejects_non_positive_key_ttl_seconds():
         TagRateLimitEntry(name="per_minute", limit=1, period_seconds=60, key_ttl_seconds=0)
 
 
+def test_tag_rate_limit_entry_rejects_key_ttl_seconds_shorter_than_period_seconds():
+    """
+    Regression test for a real bug: a key_ttl_seconds shorter than
+    period_seconds expires the bucket key before its window rolls over,
+    resetting the counter to zero mid-window and letting tagged traffic
+    exceed the configured limit.
+    """
+    with pytest.raises(ValueError):
+        TagRateLimitEntry(name="per_minute", limit=1, period_seconds=60, key_ttl_seconds=59)
+
+
 # ---------------------------------------------------------------------------
 # per-tag max_in_memory_cache_size override -- dedicated cache partitions
 # ---------------------------------------------------------------------------