| Named By: | Agassiz in 1837 |
| Time Period: | Early Carboniferous |
| Location: | Brazil - Irati Formation, Rio do Rasto Formation, England, Russia, and Scotland - Glencartholm Volcanic Beds Formation |
| Size: | Unavailable |
| Diet: | Carnivore/Piscivore |
| Fossil(s): | Several individuals but from incomplete remains |
| Classification: | | Chordata | Chondrichthyes | Elasmobranchii | Xenacanthida | |
| Also known as: | | Ctenacanthus costellatus | Ctenacanthus hybodioides | |
Sphenacanthus is an extinct genus of a chondrichtyan xenacanthiform that belongs to the Sphenacanthidae family and lived during the Carboniferous periodo in Scotland, Spain, Russia and Brazil. It lived 359 million years ago, and probably it was one of the first member of the elasmobranchians, the lineage that leads to the modern sharks. Sphenacanthus probably hunts small fishes and, unlike their modern-day relatives, its inhabited fresh water lagoons. Sphenacanthus had seven fins, two in the upper part and five in the underside, and it have a heterodont dentition and mandibles relatively long and deeper.