| Named By: | J. Leidy in 1851 |
| Time Period: | Late Cretaceous, 80.5 Ma |
| Location: | USA, Alabama |
| Size: | Unavailable |
| Diet: | Piscivore |
| Fossil(s): | Partial remains of two species |
| Classification: | | Chordata | Reptilia | Sauropterygia | Plesiosauria | |
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.