diff --git a/litellm/llms/vertex_ai/gemini/transformation.py b/litellm/llms/vertex_ai/gemini/transformation.py
index 0db1118a7b4..71f961b94c0 100644
--- a/litellm/llms/vertex_ai/gemini/transformation.py
+++ b/litellm/llms/vertex_ai/gemini/transformation.py
@@ -1082,6 +1082,25 @@ def _rewrite_google_maps_response_format(data: RequestBody) -> None:
         _rewrite_mime_type_to_response_format(generation_config)
 
 
+_SERVICE_TIER_ENUM_PREFIX = "service_tier_"
+
+
+def _transform_service_tier(
+    service_tier: str,
+    custom_llm_provider: Literal["vertex_ai", "vertex_ai_beta", "gemini"],
+) -> str:
+    """
+    Google AI Studio accepts the lowercase enum names ('standard', 'flex', 'priority'), while
+    Vertex AI validates against google.cloud.aiplatform.v1.ServiceTier, whose names are prefixed
+    and uppercased ('SERVICE_TIER_FLEX').
+    """
+    normalized = service_tier.lower().removeprefix(_SERVICE_TIER_ENUM_PREFIX)
+    tier = "standard" if normalized == "default" else normalized
+    if custom_llm_provider == LlmProviders.GEMINI:
+        return tier
+    return f"{_SERVICE_TIER_ENUM_PREFIX}{tier}".upper()
+
+
 def _transform_request_body(
     messages: List[AllMessageValues],
     model: str,
@@ -1177,10 +1196,9 @@ def _transform_request_body(
 
         if service_tier := optional_params.pop("service_tier", None):
             if isinstance(service_tier, str):
-                if service_tier.lower() == "default":
-                    data["serviceTier"] = "standard"
-                else:
-                    data["serviceTier"] = service_tier.lower()
+                data["serviceTier"] = _transform_service_tier(
+                    service_tier=service_tier, custom_llm_provider=custom_llm_provider
+                )
             else:
                 data["serviceTier"] = service_tier
 
diff --git a/tests/test_litellm/llms/vertex_ai/gemini/test_vertex_ai_gemini_transformation.py b/tests/test_litellm/llms/vertex_ai/gemini/test_vertex_ai_gemini_transformation.py
index d99c190c6e5..c717ec09eb2 100644
--- a/tests/test_litellm/llms/vertex_ai/gemini/test_vertex_ai_gemini_transformation.py
+++ b/tests/test_litellm/llms/vertex_ai/gemini/test_vertex_ai_gemini_transformation.py
@@ -1,3 +1,5 @@
+import pytest
+
 from litellm.litellm_core_utils.prompt_templates.factory import (
     convert_to_gemini_tool_call_result,
 )
@@ -197,23 +199,54 @@ def test_vertex_ai_includes_labels():
     assert result["labels"] == {"project": "test", "team": "ai"}
 
 
-def test_service_tier_forwarded_to_vertex_ai():
-    """Test that service_tier in optional_params is mapped to serviceTier in request body."""
-    messages = [{"role": "user", "content": "test"}]
-    optional_params = {"service_tier": "flex"}
-    litellm_params = {}
-
+@pytest.mark.parametrize(
+    "service_tier, expected",
+    [
+        ("flex", "SERVICE_TIER_FLEX"),
+        ("FLEX", "SERVICE_TIER_FLEX"),
+        ("priority", "SERVICE_TIER_PRIORITY"),
+        ("standard", "SERVICE_TIER_STANDARD"),
+        ("default", "SERVICE_TIER_STANDARD"),
+        ("SERVICE_TIER_FLEX", "SERVICE_TIER_FLEX"),
+    ],
+)
+def test_service_tier_forwarded_to_vertex_ai_as_proto_enum_name(service_tier, expected):
+    """
+    Vertex AI validates serviceTier against google.cloud.aiplatform.v1.ServiceTier, so anything
+    other than the prefixed, uppercased enum name is rejected with a 400
+    """
     result = _transform_request_body(
-        messages=messages,
+        messages=[{"role": "user", "content": "test"}],
         model="gemini-2.5-pro",
-        optional_params=optional_params,
+        optional_params={"service_tier": service_tier},
         custom_llm_provider="vertex_ai",
-        litellm_params=litellm_params,
+        litellm_params={},
         cached_content=None,
     )
 
-    assert "serviceTier" in result
-    assert result["serviceTier"] == "flex"
+    assert result["serviceTier"] == expected
+
+
+@pytest.mark.parametrize(
+    "service_tier, expected",
+    [
+        ("flex", "flex"),
+        ("PRIORITY", "priority"),
+        ("default", "standard"),
+    ],
+)
+def test_service_tier_forwarded_to_google_ai_studio_as_lowercase(service_tier, expected):
+    """Google AI Studio's generativelanguage API only accepts the lowercase enum names"""
+    result = _transform_request_body(
+        messages=[{"role": "user", "content": "test"}],
+        model="gemini-2.5-pro",
+        optional_params={"service_tier": service_tier},
+        custom_llm_provider="gemini",
+        litellm_params={},
+        cached_content=None,
+    )
+
+    assert result["serviceTier"] == expected
 
 
 def test_extra_body_cache_not_forwarded_to_vertex_ai():