Phthinosuchus is an extinct genus of therapsid from the Middle Permian of Russia. Phthinosuchus is the sole member of the family Phthinosuchidae. Phthinosuchus may have been one of the most primitive therapsids, meaning that its ancestors may have branched off early from the main therapsid line. Phthinosuchus was 1.5 m long with a 20 cm skull, and looked much like the Sphenacodontids, such as Dimetrodon and Sphenacodon. Its temporal fenestrae were larger than those of the Sphenacodontids. Like the other early therapsids, it was probably sprawling and carnivorous.