| Named By: | Samuel Paul Welles in 1943 |
| Time Period: | Late Cretaceous, 70-66 Ma |
| Location: | USA - California - Moreno Formation |
| Size: | Up to 13 meters long |
| Diet: | Piscivore |
| Fossil(s): | Almost complete individual |
| Classification: | | Chordata | Reptilia | Sauropterygia | Plesiosauria | Elasmosauridae | |
Hydrotherosaurus (meaning "water beast lizard") is an extinct genus of elasmosaurid plesiosaur from the Upper Cretaceous (Maastrichtian stage) of Fresno County, California, USA. The only known species, H. alexandrae, was named for its discoverer, Annie Montague Alexander, by Samuel Paul Welles.