Hydrotherosaurus (meaning "fisherman lizard") is an extinct genus of elasmosaurid plesiosaur from the Upper Cretaceous (Maastrichtian stage) Moreno Formation of Fresno County, California, USA.

The only known species, H. alexandrae, was named for Annie Montague Alexander in 1943 by Samuel Paul Welles.

Afterwards, Fresno State College and the University of California Museum of Paleontology organized a joint expedition which would uncover a nearly complete fossilized skeleton of the animal, with only parts of the skull, shoulder girdle, flippers and certain vertebral elements missing.

This specimen, designated UCMP 33912, was collected in a ravine on the property of the Sun Ray Gypsum Mine, about 22 miles west of Mendota, California, and the deposits from which it originates are part of the Moreno Formation.

It had a small head that measured about 33 centimetres (13 in; 1.08 ft) long, a streamlined body, and four large flippers that were specially designed to help the huge animal balance, move, and accelerate itself.