diff --git a/litellm/llms/bedrock/common_utils.py b/litellm/llms/bedrock/common_utils.py
index 66ee5f10679..048d023a1bd 100644
--- a/litellm/llms/bedrock/common_utils.py
+++ b/litellm/llms/bedrock/common_utils.py
@@ -758,12 +758,13 @@ def build_mantle_messages_url(
     """Build the bedrock-mantle Anthropic /messages URL.
 
     Honors an explicit endpoint override (``api_base``, then
-    ``aws_bedrock_runtime_endpoint``) so private VPC / VPCE / GovCloud Mantle
-    endpoints are reachable; otherwise falls back to the public regional host.
+    ``aws_bedrock_runtime_endpoint``, then ``BEDROCK_MANTLE_API_BASE``) so
+    private VPC / VPCE / GovCloud Mantle endpoints are reachable; otherwise
+    falls back to the public regional host.
     The mantle messages path is appended unless the override already carries it,
     so callers can pass either the host or the full messages URL.
     """
-    override: Final = api_base or aws_bedrock_runtime_endpoint
+    override: Final = api_base or aws_bedrock_runtime_endpoint or get_secret_str("BEDROCK_MANTLE_API_BASE")
     if override:
         base: Final = override.rstrip("/")
         if base.endswith(MANTLE_MESSAGES_PATH):
diff --git a/tests/test_litellm/llms/bedrock/test_mantle.py b/tests/test_litellm/llms/bedrock/test_mantle.py
index d34517f61f6..d1d1ba447fb 100644
--- a/tests/test_litellm/llms/bedrock/test_mantle.py
+++ b/tests/test_litellm/llms/bedrock/test_mantle.py
@@ -128,6 +128,12 @@ def test_mantle_messages_url_construction():
 _VPC_ENDPOINT = "https://vpce-0a1b2c3d.bedrock-mantle.us-gov-west-1.vpce.amazonaws.com"
 
 
+@pytest.fixture(autouse=True)
+def no_ambient_mantle_api_base(monkeypatch):
+    monkeypatch.delenv("BEDROCK_MANTLE_API_BASE", raising=False)
+
+
+
 def test_mantle_chat_url_honors_api_base_host():
     config = AmazonMantleConfig()
     url = config.get_complete_url(
@@ -193,6 +199,42 @@ def test_mantle_messages_url_honors_aws_bedrock_runtime_endpoint():
     assert url == f"{_VPC_ENDPOINT}/anthropic/v1/messages"
 
 
+_ENV_ENDPOINT = "https://bedrock-mantle.us-east-1.api.aws.internal.example.com"
+
+
+@pytest.mark.parametrize("config_cls", [AmazonMantleConfig, AmazonMantleMessagesConfig])
+def test_mantle_url_honors_bedrock_mantle_api_base_env(monkeypatch, config_cls):
+    monkeypatch.setenv("BEDROCK_MANTLE_API_BASE", _ENV_ENDPOINT)
+    url = config_cls().get_complete_url(
+        api_base=None,
+        api_key=None,
+        model="mantle/anthropic.claude-mythos-preview",
+        optional_params={"aws_region_name": "us-east-1"},
+        litellm_params={},
+    )
+    assert url == f"{_ENV_ENDPOINT}/anthropic/v1/messages"
+
+
+@pytest.mark.parametrize("config_cls", [AmazonMantleConfig, AmazonMantleMessagesConfig])
+@pytest.mark.parametrize(
+    ("api_base", "optional_params"),
+    [
+        (_VPC_ENDPOINT, {"aws_region_name": "us-gov-west-1"}),
+        (None, {"aws_region_name": "us-gov-west-1", "aws_bedrock_runtime_endpoint": _VPC_ENDPOINT}),
+    ],
+)
+def test_mantle_url_explicit_endpoint_beats_bedrock_mantle_api_base_env(monkeypatch, config_cls, api_base, optional_params):
+    monkeypatch.setenv("BEDROCK_MANTLE_API_BASE", _ENV_ENDPOINT)
+    url = config_cls().get_complete_url(
+        api_base=api_base,
+        api_key=None,
+        model="mantle/anthropic.claude-mythos-preview",
+        optional_params=optional_params,
+        litellm_params={},
+    )
+    assert url == f"{_VPC_ENDPOINT}/anthropic/v1/messages"
+
+
 def test_mantle_transform_request_strips_prefix_and_adds_model():
     config = AmazonMantleConfig()
     request = config.transform_request(