| Named By: | Edward Drinker Cope in 1868 |
| Time Period: | Late Cretaceous, 80.5 Ma |
| Location: | USA, particularly associated with Kansas - Pierre Shale |
| Size: | 14 meters long |
| Diet: | Piscivore |
| Fossil(s): | Many specimens |
| Classification: | | Chordata | Reptilia | Sauropterygia | Plesiosauria | Elasmosauridae | |
Elasmosaurus (from Greek elasmos elasmos 'thin plate' (referring to thin plates in its pelvic girdle) + sauros sauros 'lizard') is a genus of plesiosaur with an extremely long neck that lived in the Late Cretaceous period (Campanian stage), 80.5 million years ago.