| Named By: | H. H. Lane in 1945 |
| Time Period: | Pennsylvanian, 302 Ma |
| Location: | USA, Kansas |
| Size: | 40 centimetres long |
| Diet: | unavailable |
| Fossil(s): | Well enough preserved to reveal the full morphology |
| Classification: | | Chordata | Reptilia | Diapsida | Araeoscelida | Petrolacosauridae | |
Petrolacosaurus was a small, 40-centimetre (16 in) long reptile, and the earliest known diapsid. It is at the base of the largest reptilian radiation that includes eosuchians, thecodonts, rhynchosaurs, pterosaurs, dinosaurs, and eventually birds.