An order of herbivorous diapsid reptiles in the infraclass Archosauria, limited to the Triassic System but with a worldwide distribution. Rhynchosaurs were pig- or sheep-sized quadrupedal reptiles that were common in the Middle and Upper Triassic Series of India, South America, Europe, and eastern Africa. Except for the earliest South African genus, Mesosuchus, they are characterized by multiple rows of teeth on both the upper and lower jaws that were fused into deep sockets. Teeth were not replaced as they were worn but were added posteriorly as the jaw grew. In most genera, the premaxillae are devoid of teeth and overhang the front of the lower jaw like a beak. The external nostril is median rather than paired.

Rhynchosaurs were elements of the early archosauromorph radiation that included the protorosaurs and primitive archosaurs. The structure of the ankle is nearly identical in the early members of these groups, but later rhynchosaurs enlarged the centrals which contributed to a simple hinge joint between the lower leg and the tarsals.

Rhynchosaurs were locally common in the Late Triassic but are unknown in the Jurassic. Their rapid extinction may have resulted from changes in the vegetation on which they fed, or from predation from the expanding community of predaceous archosaurs, including early dinosaurs. See also Archosauria; Diapsida; Protorosauria; Reptilia.