| Named By: | Welles in 1943 |
| Time Period: | Late Cretaceous, 95 Ma |
| Location: | USA, Colorado - Graneros Shale Formation, and Montana - Belle Fourche Formation |
| Size: | 12 meters long |
| Diet: | Piscivore |
| Fossil(s): | Remains of 6 individuals |
| Classification: | | Chordata | Reptilia | Sauropterygia | Plesiosauria | Elasmosauridae | |
| Also known as: | | Alzadasaurus riggsi | |
Thalassomedon is a genus of plesiosaur, named by Welles in 1943. Greek, thalassa, "sea" and Greek, medon, "lord, ruler": Sea lord.