JWT Decoder

How to Use the JWT Decoder

This free online JWT decoder lets you inspect JSON Web Tokens directly in your browser. Paste any JWT into the input field and click "Decode" to instantly view the header, payload, and signature components. The tool automatically detects and displays expiration (exp) and issued-at (iat) timestamps in a human-readable format, and indicates whether the token is currently valid or expired. All decoding happens client-side, so your tokens are never transmitted to any external server.

What Is a JSON Web Token (JWT)?

A JSON Web Token is a compact, URL-safe means of representing claims to be transferred between two parties. Defined by RFC 7519, a JWT consists of three Base64URL-encoded parts separated by dots: the header, the payload, and the signature. The header typically contains the token type (JWT) and the signing algorithm (such as HS256 or RS256). The payload contains the claims, which are statements about the entity (typically the user) and additional metadata. The signature is used to verify the token has not been altered and, in the case of tokens signed with a private key, to verify the identity of the sender.

Common JWT Claims

JWTs contain registered claims such as iss (issuer), sub (subject), aud (audience), exp (expiration time), nbf (not before), iat (issued at), and jti (JWT ID). Custom claims can also be included to carry application-specific data such as user roles, permissions, or profile information. Understanding these claims is essential for debugging authentication flows, API integrations, and authorization issues in modern web applications.

Use Cases for JWT Decoding

Developers frequently need to decode JWTs when debugging authentication systems, verifying token contents during API development, inspecting OAuth2 and OpenID Connect tokens, troubleshooting session management issues, and validating token expiration in frontend applications. This tool provides a quick and secure way to inspect token contents without installing any software or using command-line tools. Note that this tool only decodes the token; it does not verify the signature, which requires the signing key.

JWTデコーダーの使い方

この無料オンラインJWTデコーダーは、JSON Web Tokenをブラウザ内で即座にデコードします。 入力欄にJWTトークンを貼り付け、「デコード」ボタンをクリックするだけで、 ヘッダー、ペイロード、署名を確認できます。有効期限(exp)や発行日時(iat)が含まれている場合は 自動的に日本時間で表示され、トークンが有効か期限切れかも判定されます。 全ての処理はブラウザ内で完結し、トークンが外部サーバーに送信されることはありません。