diff --git a/src/mcp/server/auth/middleware/bearer_auth.py b/src/mcp/server/auth/middleware/bearer_auth.py
index 295605af7..30b5e2ba6 100644
--- a/src/mcp/server/auth/middleware/bearer_auth.py
+++ b/src/mcp/server/auth/middleware/bearer_auth.py
@@ -34,8 +34,15 @@ def __init__(
         self.provider = provider
 
     async def authenticate(self, conn: HTTPConnection):
-        auth_header = conn.headers.get("Authorization")
-        if not auth_header or not auth_header.startswith("Bearer "):
+        auth_header = next(
+            (
+                conn.headers.get(key)
+                for key in conn.headers
+                if key.lower() == "authorization"
+            ),
+            None,
+        )
+        if not auth_header or not auth_header.lower().startswith("bearer "):
             return None
 
         token = auth_header[7:]  # Remove "Bearer " prefix
diff --git a/tests/server/auth/middleware/test_bearer_auth.py b/tests/server/auth/middleware/test_bearer_auth.py
index 9acb5ff09..e8c17a4c4 100644
--- a/tests/server/auth/middleware/test_bearer_auth.py
+++ b/tests/server/auth/middleware/test_bearer_auth.py
@@ -7,6 +7,7 @@
 
 import pytest
 from starlette.authentication import AuthCredentials
+from starlette.datastructures import Headers
 from starlette.exceptions import HTTPException
 from starlette.requests import Request
 from starlette.types import Message, Receive, Scope, Send
@@ -221,6 +222,66 @@ async def test_token_without_expiry(
         assert user.access_token == no_expiry_access_token
         assert user.scopes == ["read", "write"]
 
+    async def test_lowercase_bearer_prefix(
+        self,
+        mock_oauth_provider: OAuthAuthorizationServerProvider[Any, Any, Any],
+        valid_access_token: AccessToken,
+    ):
+        """Test with lowercase 'bearer' prefix in Authorization header"""
+        backend = BearerAuthBackend(provider=mock_oauth_provider)
+        add_token_to_provider(mock_oauth_provider, "valid_token", valid_access_token)
+        headers = Headers({"Authorization": "bearer valid_token"})
+        scope = {"type": "http", "headers": headers.raw}
+        request = Request(scope)
+        result = await backend.authenticate(request)
+        assert result is not None
+        credentials, user = result
+        assert isinstance(credentials, AuthCredentials)
+        assert isinstance(user, AuthenticatedUser)
+        assert credentials.scopes == ["read", "write"]
+        assert user.display_name == "test_client"
+        assert user.access_token == valid_access_token
+
+    async def test_mixed_case_bearer_prefix(
+        self,
+        mock_oauth_provider: OAuthAuthorizationServerProvider[Any, Any, Any],
+        valid_access_token: AccessToken,
+    ):
+        """Test with mixed 'BeArEr' prefix in Authorization header"""
+        backend = BearerAuthBackend(provider=mock_oauth_provider)
+        add_token_to_provider(mock_oauth_provider, "valid_token", valid_access_token)
+        headers = Headers({"authorization": "BeArEr valid_token"})
+        scope = {"type": "http", "headers": headers.raw}
+        request = Request(scope)
+        result = await backend.authenticate(request)
+        assert result is not None
+        credentials, user = result
+        assert isinstance(credentials, AuthCredentials)
+        assert isinstance(user, AuthenticatedUser)
+        assert credentials.scopes == ["read", "write"]
+        assert user.display_name == "test_client"
+        assert user.access_token == valid_access_token
+
+    async def test_mixed_case_authorization_header(
+        self,
+        mock_oauth_provider: OAuthAuthorizationServerProvider[Any, Any, Any],
+        valid_access_token: AccessToken,
+    ):
+        """Test authentication with mixed 'Authorization' header."""
+        backend = BearerAuthBackend(provider=mock_oauth_provider)
+        add_token_to_provider(mock_oauth_provider, "valid_token", valid_access_token)
+        headers = Headers({"AuThOrIzAtIoN": "BeArEr valid_token"})
+        scope = {"type": "http", "headers": headers.raw}
+        request = Request(scope)
+        result = await backend.authenticate(request)
+        assert result is not None
+        credentials, user = result
+        assert isinstance(credentials, AuthCredentials)
+        assert isinstance(user, AuthenticatedUser)
+        assert credentials.scopes == ["read", "write"]
+        assert user.display_name == "test_client"
+        assert user.access_token == valid_access_token
+
 
 @pytest.mark.anyio
 class TestRequireAuthMiddleware: