RADIUS (Remote Authentication Dial-In User Service) is a protocol for checking a user's authentication and authorization information from a remote server. It is originally intended for authenticating dial-in users, but is also suitable for use with Secure Shell. In SSH Tectia, RADIUS is implemented as a submethod of keyboard-interactive authentication.

When using RADIUS authentication, SSH Tectia Server first asks the user's password and then sends it along with the username to the RADIUS server (PAP authentication). Multiple RADIUS servers can be configured, and these will be queried in turn in case some of them are unreachable.

The supported RADIUS servers are Microsoft IAS (Internet Authentication Service) and FreeRADIUS.