Tappenosaurus ("Tappen's lizard") is an extinct genus of synapsids from the Middle Permian of Texas.

American paleontologists Everett C. Olson and James Beerbower described the genus in 1953 based on three specimens that were uncovered from the San Angelo Formation.

These bones are larger than comparable parts of the largest skeletons of Dimetrodon, a closely related and much better known sail-backed synapsid.

Olson and Beerbower designated the type species Tappenosaurus magnus in reference to its large size, and also placed it in its own family, Tappenosauridae.

[1] Olson later estimated the total length of Tappenosaurus to be 18 feet (5.5 m), comparing it in size with the largest of the dinocephalians, a more advanced group of synapsids that lived later in the Permian.