| Named By: | Carrol in 1981 |
| Time Period: | Changhsingian - Induan 253.8-249.7 Ma |
| Location: | Madagascar |
| Size: | 60 centimetres long |
| Diet: | unavailable |
| Fossil(s): | Several specimens, but the skull is only known from a juvenile |
| Classification: | | Chordata | Sauropsida | Eosuchia | Tangasauridae | |
Thadeosaurus is an extinct genus of diapsid reptile belonging to the family Younginidae. Fossils have been found in Madagascar, and date to the late Permian to the early Triassic period.
Thadeosaurus was a lizard-like animal, with a remarkably long tail that comprised about two thirds of the animal's total length of 60 centimetres (24 in). It also had long toes, especially on the hind legs, which would have given it a powerful stride, since the toes would still touch the ground while the foot was being raised. Combined with a strong breast bone to increase the strength in the forelimbs, this means that Thadeosaurus was probably a good runner.
Below is a cladogram from Reisz et al. (2011) showing the phylogenetic position of Thadeosaurus among other early diapsids: