| Named By: | Samuel Paul Welles in 1943 |
| Time Period: | Late Cretaceous, 70-66 Ma |
| Location: | USA - California - Moreno Formation |
| Size: | Estimate around 10 meters long for the holotype, but possibly up to 12 meters when fully grown |
| Diet: | Piscivore |
| Fossil(s): | Partial skeleton of a juvenile/subadult |
| Classification: | | Chordata | Reptilia | Sauropterygia | Plesiosauria | Elasmosauridae | |
Fresnosaurus is an extinct genus of plesiosaur from the Late Cretaceous (Maastrichtian stage) of what is now California. The type species is Fresnosaurus drescheri, first described by Welles in 1943. The generic name Fresnosaurus honors Fresno County, while the specific name honors Arthur Drescher.
Fresnosaurus was probably at least 30 feet in length. Like all elasmosaurid plesiosaurs, it probably ate small bony fish, belemnites, and ammonites.