Clerk is an authentication and user management service made by Clerk, Inc. Authentication checks who a user is at sign-in; user management covers the accounts, profiles and sessions after that. A developer adds Clerk through SDKs (software development kits) for frameworks such as Next.js, React and Expo, and drops in prebuilt sign-in, sign-up and profile components.
Clerk supports passwords checked against leaked-password databases, email and SMS one-time codes, magic links, passkeys, multifactor authentication and more than 20 social sign-in providers such as Google. It manages sessions, blocks fraudulent sign-ups, and adds organizations with roles for business-to-business apps. Clerk can also act as an OAuth authorization server: other apps, command-line tools and Model Context Protocol (MCP) clients receive scoped access tokens after the user approves a consent screen. Public clients use PKCE (Proof Key for Code Exchange), which protects the sign-in flow of a client that cannot keep a secret.
It is a hosted service. The Hobby plan is free up to 50,000 monthly retained users per app, users who return at least 24 hours after signing up; paid plans add a monthly fee and a price for each further user.