| Named By: | Samuel Paul Welles in 1943 |
| Time Period: | Late Cretaceous, 83.5-80.5 Ma |
| Location: | USA, Kansas, and South Dakota |
| Size: | 11 to 12 meters long |
| Diet: | Piscivore |
| Fossil(s): | At least two specimens |
| Classification: | | Chordata | Reptilia | Sauropterygia | Plesiosauria | Elasmosauridae | |
| Also known as: | | Alzadasaurus kansasensis | Alzadasaurus pembertonii | Cimoliasaurus snowii | Elasmosaurus snowii | Thalassiosaurus ischiadicus | Thalassonomosaurus marshi | |
Styxosaurus is a genus of plesiosaur of the family Elasmosauridae. Styxosaurus lived during the Santonian and Campanian ages of the Cretaceous period.