Trilophosaurus (Greek for "Lizard with three ridges") was a lizard-like Triassic diapsid reptile related to the archosaurs. It was a herbivore up to 2.5 m long. It had a short, unusually heavily-built skull, equipped with massive, broad flattened cheek teeth with sharp shearing surfaces for cutting up tough plant material. Teeth are absent from the premaxilla and front of the lower jaw, which in life were probably equipped with a horny beak.
The skull is also unusual in that the lower temporal opening is missing, giving the appearance of a euryapsid skull. Because of this, the trilophosaurs were once classified with placodonts within Sauropterygia. Carroll (1988) suggests that the lower opening may have been lost to strengthen the skull.
Trilophosaurus is far known only from the Late Triassic of North America and Europe.