Theretairus is a Late Jurassic genus of sphenodont reptile from the Morrison Formation of western North America,[1] present in stratigraphic zones 5 and 6.

[2] The holotype consists of a right mandible and several in-socket teeth from Quarry 9 at Como Bluff, Wyoming where it was excavated by paleontologist William Harlow Reed, who then was working for Othniel Charles Marsh,[1][3] and it was deposited at the Yale Peabody Museum under YPM VP 13764.

[3] The locality comes from strata of the Brushy Basin Member of the Morrison Formation, which also bears other sphenodontians Opisthias and Eilenodon.

[3] The type specimen preserves a very strange morphology, with masseteric fossae in the posterior end similar to those in mammals.

The anterior border of the symphysis is steeply inclined in the holotype of Theretarius, suggesting it is an adult or older individual.