diff --git a/litellm/proxy/auth/oauth2_check.py b/litellm/proxy/auth/oauth2_check.py
index 09bcb7589d1..23716c81fde 100644
--- a/litellm/proxy/auth/oauth2_check.py
+++ b/litellm/proxy/auth/oauth2_check.py
@@ -143,14 +143,18 @@ class Oauth2Handler:
                 # OAuth2 Token Introspection (RFC 7662) - requires POST with form data
                 verbose_proxy_logger.debug("Using OAuth2 introspection endpoint (POST)")
 
-                headers, data = Oauth2Handler._prepare_introspection_request(
-                    token=token,
-                    oauth_client_id=oauth_client_id,
-                    oauth_client_secret=oauth_client_secret,
+                headers, introspection_body = (
+                    Oauth2Handler._prepare_introspection_request(
+                        token=token,
+                        oauth_client_id=oauth_client_id,
+                        oauth_client_secret=oauth_client_secret,
+                    )
                 )
 
                 response = await client.post(
-                    token_info_endpoint, headers=headers, data=data
+                    token_info_endpoint,
+                    headers=headers,
+                    content=introspection_body,
                 )
             else:
                 # Generic token info endpoint - uses GET with Bearer token
diff --git a/tests/test_litellm/proxy/auth/test_oauth2_check.py b/tests/test_litellm/proxy/auth/test_oauth2_check.py
index d4f21a01b22..37241ce30df 100644
--- a/tests/test_litellm/proxy/auth/test_oauth2_check.py
+++ b/tests/test_litellm/proxy/auth/test_oauth2_check.py
@@ -1,8 +1,11 @@
 import base64
 import os
 import sys
+from unittest.mock import AsyncMock, MagicMock, patch
 from urllib.parse import parse_qs
 
+import pytest
+
 sys.path.insert(0, os.path.abspath("../../../.."))
 
 from litellm.proxy.auth.oauth2_check import Oauth2Handler
@@ -47,3 +50,41 @@ def test_introspection_request_without_credentials_only_sends_token():
     assert "Authorization" not in headers
     parsed = parse_qs(body)
     assert parsed == {"token": ["opaque-token"]}
+
+
+@pytest.mark.asyncio
+async def test_check_oauth2_token_sends_introspection_body_as_raw_content(monkeypatch):
+    """The introspection POST must send the pre-encoded form body via httpx
+    ``content=``. ``_prepare_introspection_request`` already form-encodes the
+    body (and Authlib may append client auth), so passing it through ``data=``
+    would re-encode it and is deprecated for strings in httpx.
+    """
+    monkeypatch.setenv(
+        "OAUTH_TOKEN_INFO_ENDPOINT", "https://idp.example.com/introspect"
+    )
+    monkeypatch.setenv("OAUTH_CLIENT_ID", "client-id")
+    monkeypatch.setenv("OAUTH_CLIENT_SECRET", "client-secret")
+
+    response = MagicMock()
+    response.raise_for_status = MagicMock()
+    response.json = MagicMock(
+        return_value={"active": True, "sub": "u1", "role": "internal_user"}
+    )
+
+    client = MagicMock()
+    client.post = AsyncMock(return_value=response)
+
+    with (
+        patch("litellm.proxy.proxy_server.premium_user", True),
+        patch(
+            "litellm.proxy.auth.oauth2_check.get_async_httpx_client",
+            return_value=client,
+        ),
+    ):
+        await Oauth2Handler.check_oauth2_token("opaque-token")
+
+    assert client.post.await_count == 1
+    kwargs = client.post.await_args.kwargs
+    assert "data" not in kwargs
+    assert isinstance(kwargs["content"], str)
+    assert "token=opaque-token" in kwargs["content"]