A key agreement protocol is a protocol whereby two or more communicating parties can agree on a key or exchange information over an open communication network in such a way that both of them agree on the established session keys for use in subsequent communications. This paper proposes an efficient anonymous key agreement protocol based on chaotic maps. By utilizing a legitimate pseudonym mechanism, a trusted third party can verify the identity of the user in an efficient way. Therefore, the proposed protocol not only greatly reduces computational costs between communication parties, but also allows users to anonymously interact with the server. Moreover, our analysis shows that the proposed protocol can successfully defend Bergamo et al.'s attack, replay attacks, man-in-the-middle attacks, and stolen-verifier attacks.