| Named By: | S. G. Hunt & A. P. Hunt in 1990 |
| Time Period: | Late Triassic, 225 Ma |
| Location: | USA, Texas - Tecovas Formation |
| Size: | Uncertain due to lack of remains, Skull length about 17 millimetres |
| Diet: | Insectivore |
| Fossil(s): | Partial skull |
| Classification: | | Chordata | Mammaliamorpha | |
Adelobasileus cromptoni is a species of an extinct genus of mammal-like synapsid from the Late Triassic (Carnian), about 225 million years ago. It is known only from a partial skull recovered from the Tecovas formation in western Texas, southern United States.
Roughly contemporary with the mammaliaform Tikitherium, Adelobasileus predates the non-mammalian cynodonts Tritylodontidae and Tritheledontidae by 10 million years. In fact, distinct cranial features, especially the housing of the cochlea, suggest that Adelobasileus is a transitional form in the character transformation from cynodonts to Triassic mammals. For this reason, it is thought to be the common ancestor of all modern mammals or a close relative of the common ancestor. Though traditionally classified as a mammal by trait-based taxonomy, it is outside the crown group containing all true mammals.