The pelycosaurs were smallish to large (up to 3 meters or more) primitive Upper Paleozoic synapsid reptiles. They appeared during the Upper Carboniferous and reached their acme in the Lower Permian, remaining the dominant land animals for many millions of years. A few stragglers continued into the late Permian.

At least two pelycosaur clades independently evolved a tall sail, consisting of elongated vertebral spines: the edaphosaurids and the sphenacodontids . In life this would have been covered by skin, and presumably functioned as a thermoregulatory device. Pelycosaur fossils have been found mainly in Europe and North America, although some small late-surviving forms are known from Russia and South Africa.

Well-known pelycosaurs include the genera Dimetrodon, Sphenacodon , Edaphosaurus , and Ophiacodon .

Pelycosauria is a paraphyletic taxon because it excludes the therapsids. Eupelycosauria is used for Pelycosauria and Therapsida, and is monophyletic.