HTTP 404或Not Found錯誤訊息是HTTP的其中一種「標準回應訊息」（HTTP狀態碼），此訊息代表客戶端在瀏覽網頁時，伺服器無法正常提供訊息，或是伺服器無法回應且不知原因。通常是因為用戶所存取的對應網頁已被刪除、移動或從未存在。404也是互聯網上最常見的錯誤之一。404錯誤訊息可能與「server not found」（無法找到伺服器）或其他類似訊息產生混淆。
The HTTP 404, 404 Not Found, 404, 404 Error, Page Not Found, File Not Found, or Server Not Found error message is a Hypertext Transfer Protocol (HTTP) standard response code, in computer network communications, to indicate that the browser was able to communicate with a given server, but the server could not find what was requested. The error may also be used when a server does not wish to disclose whether it has the requested information.