| Named By: | Eleanor Daly in 1969 |
| Time Period: | Early Permian, 273.6-271.6 Ma |
| Location: | USA, Oklahoma |
| Size: | Total body length is unknown but the skull was 3.5 centimetres long |
| Diet: | unavailable |
| Fossil(s): | Skull |
| Classification: | | Chordata | Reptilia | Anapsida | Parareptilia | Acleistorhinidae | |
Acleistorhinus (ah-kles-toe-RYE-nuss) is an extinct genus of parareptile known from the Early Permian (middle Kungurian stage) of Oklahoma It is notable for being the earliest known anapsid reptile yet discovered. The morphology of the lower temporal fenestra of the skull of Acleistorhinus bears a superficial resemblance to that seen in early synapsids, the result of convergent evolution. Only a single species, A. pteroticus, is known, and it is classified in the Family Acleistorhinidae, along with Colobomycter (also from the Early Permian of Oklahoma).