| Named By: | Henry Riley & Samuel Stutchbury in 1836 |
| Time Period: | Late Triassic-Early Jurassic,[1] 203.6-199.6 Ma |
| Location: | United Kingdom, Southern England. France |
| Size: | Average of 1.2 meteres long, but some individuals estimated up to 2.5 meters long |
| Diet: | unavailable |
| Fossil(s): | Multiple post cranial remains and some skull material |
| Classification: | | Chordata | Reptilia | Dinosauria | Saurischia | Sauropodomorpha | |
| Also known as: | | Agrosaurus macgillivrayi | |
Thecodontosaurus ("socket-tooth lizard") is a genus of herbivorous basal sauropodomorph dinosaur that lived during the late Triassic period (Rhaetian age).
Its remains are known mostly from Triassic "fissure fillings" in South England. Thecodontosaurus was a small bipedal animal, about 2 m (6.5 ft) long. It is one of the first dinosaurs that were discovered and one of the oldest that existed. Many species have been named in the genus, but only the type species Thecodontosaurus antiquus is seen as valid today.