diff --git a/src/mcp/server/streamable_http_manager.py b/src/mcp/server/streamable_http_manager.py
index 53d542d21..e45db667f 100644
--- a/src/mcp/server/streamable_http_manager.py
+++ b/src/mcp/server/streamable_http_manager.py
@@ -273,7 +273,7 @@ async def run_server(*, task_status: TaskStatus[None] = anyio.TASK_STATUS_IGNORE
         else:
             # Invalid session ID
             response = Response(
-                "Bad Request: No valid session ID provided",
-                status_code=HTTPStatus.BAD_REQUEST,
+                "Not Found: Unknown session ID",
+                status_code=HTTPStatus.NOT_FOUND,
             )
             await response(scope, receive, send)
diff --git a/tests/server/test_streamable_http_manager.py b/tests/server/test_streamable_http_manager.py
index 7a8551e5c..747bd8c1f 100644
--- a/tests/server/test_streamable_http_manager.py
+++ b/tests/server/test_streamable_http_manager.py
@@ -1,5 +1,6 @@
 """Tests for StreamableHTTPSessionManager."""
 
+from http import HTTPStatus
 from typing import Any
 from unittest.mock import AsyncMock, patch
 
@@ -202,6 +203,59 @@ async def mock_receive():
     assert not manager._server_instances, "No sessions should be tracked after the only session crashes"
 
 
+@pytest.mark.anyio
+async def test_stateful_session_returning_http_not_found_when_not_found(
+    running_manager: tuple[StreamableHTTPSessionManager, Server],
+):
+    """Test that a request with a non-existent session ID returns HTTP 404 Not Found.
+
+    This is in accordance to the specification point 2.5.3 of:
+    https://modelcontextprotocol.io/specification/2025-06-18/basic/transports
+    """
+    manager, app = running_manager
+
+    mock_mcp_run = AsyncMock(return_value=None)
+    app.run = mock_mcp_run
+
+    sent_messages: list[Message] = []
+
+    async def mock_send(message: Message):
+        sent_messages.append(message)
+
+    scope = {
+        "type": "http",
+        "method": "POST",
+        "path": "/mcp",
+        "headers": [
+            (b"content-type", b"application/json"),
+            # The point of this test -- non-existent session ID:
+            (b"mcp-session-id", b"non-existent-session-id"),
+        ],
+    }
+
+    async def mock_receive():
+        return {"type": "http.request", "body": b"", "more_body": False}
+
+    # Send the request with mcp-session-id header set to a non-existent value.
+    await manager.handle_request(scope, mock_receive, mock_send)
+
+    # Extract HTTP status and body from the messages.
+    http_status = None
+    body = None
+    for msg in sent_messages:
+        if msg["type"] == "http.response.start":
+            http_status = msg["status"]
+            break
+    for msg in sent_messages:
+        if msg["type"] == "http.response.body":
+            body = msg["body"]
+            break
+
+    assert http_status == HTTPStatus.NOT_FOUND, "Response status should be 404 Not Found"
+    assert body is not None, "Response body should not be None"
+    assert b"Not Found" in body, "Response body should indicate Not Found"
+
+
 @pytest.mark.anyio
 async def test_stateless_requests_memory_cleanup():
     """Test that stateless requests actually clean up resources using real transports."""