Sphenacodon (meaning "wedge point tooth") is an extinct genus of synapsid that lived from about 300 to about 280 million years ago (Ma) during the Late Carboniferous and Early Permian periods.

Sphenacodon and Dimetrodon typically have been found in different geographical areas that were separated by the ancient Hueco Seaway that penetrated equatorial Pangaea during the Early Permian and "covered much of southern New Mexico and parts of West Texas".

In Dimetrodon, the neural spines develop into long, narrow, cylindrical projections that support a tall vertical dorsal sail that ends near the base of the tail.

There is evidence for strong epaxial muscles along the base of the raised neural spines in both Sphenacodon and Dimetrodon, likely helping to stiffen and strengthen the backbone for walking and for lunging at prey by restricting side-to-side flexing motion.

A recent study[8] of the structure of the neural spines on Sphenacodon confirms that the upper parts were not encased in a thick muscular hump and instead protruded above a layer of muscle to form a low dorsal crest.

[9] Both Sphenacodon and Dimetrodon have been depicted with their short limbs splayed outward at 90 degrees from the body in a wide pushup position and with the tail (and even belly) dragging on the ground, similar to modern lizards and crocodiles.

[10] Some well preserved narrow Dimetropus tracks found in parts of the Prehistoric Trackways National Monument in New Mexico match the smaller size of Sphenacodon, a genus known from skeletal fossils in the state, but could also come from a small Dimetrodon.

The American paleontologist O. C. Marsh[11] named Sphenacodon (from Greek sphen "wedge" + ake "point" + odous (-odon) "tooth") in 1878, based on part of a lower jaw (dentary) bone found in the redbeds of northern New Mexico by fossil collector David Baldwin.

In his very short description of the jaw, Marsh cited the back teeth as characteristic ("crowns are much compressed, and have very sharp cutting edges without crenulations") and assessed the animal as "about six feet in length, and carnivorous in habit," although the rest of the skeleton was not known.

Marsh gave the genus the Latin specific name ferox "fierce" and erected the new family Sphenacodontidae, placed under the primitive reptilian order "Rhynchocephala" (=Rhynchocephalia), then including nearly all groups of early reptiles in addition to the living tuatara.