| Named By: | Agassiz in 1837 |
| Time Period: | Early Carboniferous |
| Location: | Scotland |
| Size: | 60 centimetres long |
| Diet: | Carnivore |
| Fossil(s): | Fin spines |
| Classification: | | Chordata | Chondrichthyes | Elasmobranchii | Ctenacanthida | |
Tristychius is an extinct genus of shark from the Carboniferous period. Fossils have been found in Scotland.
Tristychius was a small shark, about 60 centimetres (2 ft) long. It had a well-developed upturned caudal fin, similar to that of many modern sharks. Physically it may have resembled a modern dogfish. Tristychius also had spikes attached to the bases of its dorsal fins, probably for protection against predators.