Petrolacosaurus ("rock lake lizard") is an extinct genus of sauropsid from the late Carboniferous period. It was a small, 40-centimetre (16 in) long reptile, and one of the earliest known reptiles with two temporal fenestrae. This means that it was near the base of Diapsida (it may have been the basal taxon), the largest and most successful radiation of reptiles that would eventually include all modern reptile groups, as well as dinosaurs (which survived to the modern day as birds) and other famous extinct reptiles such as plesiosaurs, ichthyosaurs, and pterosaurs. However, Petrolacosaurus itself was part of Araeoscelidia, a short-lived early branch of the sauropsid family tree which went extinct prior to the middle Permian.

The first Petrolacosaurus fossil was found in 1932 in Garnett, Kansas, by a field expedition from the University of Kansas Natural History Museum. The party consisted of Henry H. Lane, Claude Hibbard, David Dunkle, Wallace Lane, Louis Coghill, and Curtis Hesse. Unfortunately, no field notes or documentation of their discovery are available.

Petrolacosaurus fossils were found preserved within a layer of laminated shale that was also plant bearing. The strata that the remains were found in the middle Gzhelian age (middle Stephanian age on the European scale) of the Upper Pennsylvanian.

Specimens reveal that Petrolacosaurus had a slightly elongated skull with two temporal fenestrae. The upper temporal fenestra is located posteriorly to an enlarged orbit. This gives Petrolacosaurus an anatomically diapsid skull, a feature that is convergently shared with neodiapsid reptiles.

The largest teeth in the jaw were at the front of the snout, erupting from the premaxilla bone. There were also a few larger-than average teeth further back on the maxilla, which were probably homologous with caniform teeth retained from the common ancestor between true sauropsid reptiles and synapsids (mammal ancestors). On the dentary, there are marginal teeth, displaying a primitive type of shallow implantation. The palatal arrangement bears close resemblance to Youngoides.

There are seven elongated cervical vertebrae, 2 sacrals, and 60 caudal vertebrae. The number and placement of the vertebrae show that Petrolacosaurus was a rather long-necked reptile with a shorter torso. The number and spool-like shape of the vertebrae, in addition to the poses that the reptiles died in, show that they were also very flexible creatures.

The vertebral centra were amphicoelous (concave from the front and behind) and had large ventrolateral depressions. A similar depression can be seen on the massive neural arches, running from the prezygapophyses to the postzygapophysis. These depressions allowed the vertebral column to be lighter while still retaining a sturdy build.

The pectoral girdle in Petrolacosaurus is light in comparison to the massive girdles found in taxa such as pelycosaurs. Petrolacosaurus specimens had a clavicle with an expanded ventral blade, yet shorter than that of Captorhinus or pelycosaurs. Also, there is a large cleithrum with a broad, rounded end.