Techniques and devices for seamless authentication using radar are described. In some implementations, a radar field is provided through a radar-based authentication system. The radar-based authentication system can sense reflections from an object in the radar field and analyze the reflections to determine whether the object is a person. In response to determining that the object is a person, the radar-based authentication system can sense an identifying characteristic associated with the person. Based on the identifying characteristic, the radar-based authentication system can determine that the person is an authorized user.