Cynosphenodon (/ˌsaɪnəˈsfɛnədɒn/ SY-nə-SFEN-ə-don; "Dog Sphenodontian") is an extinct genus of rhynchocephalian in the family Sphenodontidae from the Middle Jurassic La Boca Formation of Tamaulipas, Mexico.

It is known from a largely complete lower jaw and fragments of the upper jaw.

It is suggested to be among the closest known relatives of the tuatara, with both being placed in the Sphenodontinae,[1] which is supported by among other characters, the growth pattern of the teeth.

Cladogram after Simoes et al.

This article related to a Jurassic reptile is a stub.