diff --git a/litellm/proxy/guardrails/guardrail_registry.py b/litellm/proxy/guardrails/guardrail_registry.py
index 1f2c9e0c182..5c9f832cc18 100644
--- a/litellm/proxy/guardrails/guardrail_registry.py
+++ b/litellm/proxy/guardrails/guardrail_registry.py
@@ -384,6 +384,23 @@ class GuardrailRegistry:
             raise Exception(f"Error getting guardrail from DB: {str(e)}")
 
 
+def _resolve_env_references(litellm_params: LitellmParams) -> LitellmParams:
+    """Expand every ``os.environ/NAME`` reference in a guardrail's litellm_params.
+
+    Only ``api_key`` and ``api_base`` used to be expanded, so a guardrail
+    configured with e.g. ``aws_region_name: os.environ/AWS_REGION`` handed boto3
+    the literal string and failed with "Invalid AWS region format". A name that
+    resolves to nothing becomes None rather than the string "None", so the
+    downstream client falls back to its own credential chain.
+    """
+    resolved = {
+        name: get_secret(value)
+        for name, value in litellm_params.model_dump().items()
+        if isinstance(value, str) and value.startswith("os.environ/")
+    }
+    return litellm_params.model_copy(update=resolved) if resolved else litellm_params
+
+
 class InMemoryGuardrailHandler:
     """
     Class that handles initializing guardrails and adding them to the CallbackManager
@@ -442,11 +459,7 @@ class InMemoryGuardrailHandler:
             lakera_category_thresholds = LakeraCategoryThresholds(**litellm_params_data["category_thresholds"])
             litellm_params.category_thresholds = lakera_category_thresholds
 
-        if litellm_params.api_key and litellm_params.api_key.startswith("os.environ/"):
-            litellm_params.api_key = str(get_secret(litellm_params.api_key))
-
-        if litellm_params.api_base and litellm_params.api_base.startswith("os.environ/"):
-            litellm_params.api_base = str(get_secret(litellm_params.api_base))
+        litellm_params = _resolve_env_references(litellm_params)
 
         guardrail_type = litellm_params.guardrail
 
diff --git a/tests/test_litellm/proxy/guardrails/test_guardrail_registry.py b/tests/test_litellm/proxy/guardrails/test_guardrail_registry.py
index 26feddadf79..6bec0c7f6ca 100644
--- a/tests/test_litellm/proxy/guardrails/test_guardrail_registry.py
+++ b/tests/test_litellm/proxy/guardrails/test_guardrail_registry.py
@@ -367,3 +367,37 @@ def test_repeated_db_sync_does_not_accumulate_runner_instances():
     finally:
         for cb_list, snapshot in zip(lists, snapshots):
             cb_list[:] = snapshot
+
+
+def test_resolve_env_references_expands_every_string_field(monkeypatch):
+    """Regression: only api_key/api_base used to be expanded, so a guardrail set up
+    with aws_region_name="os.environ/AWS_REGION" handed boto3 the literal string and
+    failed with "Invalid AWS region format"."""
+    from litellm.proxy.guardrails.guardrail_registry import _resolve_env_references
+
+    monkeypatch.setenv("TEST_GUARDRAIL_REGION", "us-east-1")
+    monkeypatch.setenv("TEST_GUARDRAIL_KEY", "sk-guardrail")
+    monkeypatch.setenv("TEST_GUARDRAIL_ANALYZER", "https://analyzer.example")
+    monkeypatch.delenv("TEST_GUARDRAIL_ABSENT", raising=False)
+
+    resolved = _resolve_env_references(
+        LitellmParams(
+            guardrail="bedrock",
+            mode="pre_call",
+            api_key="os.environ/TEST_GUARDRAIL_KEY",
+            aws_region_name="os.environ/TEST_GUARDRAIL_REGION",
+            presidio_analyzer_api_base="os.environ/TEST_GUARDRAIL_ANALYZER",
+            aws_secret_access_key="os.environ/TEST_GUARDRAIL_ABSENT",
+            aws_access_key_id="AKIA_LITERAL_VALUE",
+        )
+    )
+
+    assert resolved.aws_region_name == "us-east-1"
+    assert resolved.presidio_analyzer_api_base == "https://analyzer.example"
+    assert resolved.api_key == "sk-guardrail"
+    # An unset name must not become the string "None", which boto3 would treat as a credential.
+    assert resolved.aws_secret_access_key is None
+    # Literal values are left alone, and unrelated fields survive the copy.
+    assert resolved.aws_access_key_id == "AKIA_LITERAL_VALUE"
+    assert resolved.guardrail == "bedrock"
+    assert resolved.mode == "pre_call"