diff --git a/langfuse/_utils/serializer.py b/langfuse/_utils/serializer.py
index 46188fce5..ac3063237 100644
--- a/langfuse/_utils/serializer.py
+++ b/langfuse/_utils/serializer.py
@@ -13,7 +13,7 @@
 from typing import Any
 from uuid import UUID
 
-from pydantic import BaseModel
+from pydantic import BaseModel, SecretBytes, SecretStr
 
 from langfuse.media import LangfuseMedia, LangfuseMediaReference
 
@@ -52,6 +52,9 @@ def default(self, obj: Any) -> Any:
 
     def _default_inner(self, obj: Any) -> Any:
         try:
+            if isinstance(obj, (SecretStr, SecretBytes)):
+                return "<secret>"
+
             if isinstance(obj, (datetime)):
                 # Timezone-awareness check
                 return serialize_datetime(obj)
diff --git a/tests/unit/test_serializer.py b/tests/unit/test_serializer.py
index ce5798f67..48e6fc292 100644
--- a/tests/unit/test_serializer.py
+++ b/tests/unit/test_serializer.py
@@ -7,7 +7,7 @@
 from uuid import UUID
 
 import pytest
-from pydantic import BaseModel
+from pydantic import BaseModel, SecretBytes, SecretStr
 
 from langfuse._utils.serializer import (
     EventSerializer,
@@ -29,6 +29,11 @@ class TestBaseModel(BaseModel):
     field: str
 
 
+class SecretBaseModel(BaseModel):
+    api_key: SecretStr
+    token: SecretBytes
+
+
 def test_datetime():
     dt = datetime(2023, 1, 1, 12, 0, 0, tzinfo=timezone.utc)
     serializer = EventSerializer()
@@ -71,6 +76,32 @@ def test_pydantic_model():
     assert json.loads(serializer.encode(model)) == {"field": "test"}
 
 
+@pytest.mark.parametrize(
+    "secret",
+    [
+        SecretStr("not-a-real-api-key"),
+        SecretBytes(b"not-a-real-token"),
+    ],
+)
+def test_pydantic_secret(secret):
+    serializer = EventSerializer()
+
+    assert serializer.encode(secret) == '"<secret>"'
+
+
+def test_pydantic_model_with_secrets():
+    model = SecretBaseModel(
+        api_key=SecretStr("not-a-real-api-key"),
+        token=SecretBytes(b"not-a-real-token"),
+    )
+    serializer = EventSerializer()
+
+    assert json.loads(serializer.encode(model)) == {
+        "api_key": "<secret>",
+        "token": "<secret>",
+    }
+
+
 def test_langfuse_media_reference_serializes_to_reference_string():
     # Resolved references must round-trip back to their original reference string
     # rather than falling through to asdict() and emitting an opaque dict.