Fresnosaurus is an extinct plesiosaur genus that lived during the Late Cretaceous, specifically in the Maastrichtian stage, within the region that is now California. The first described species within this genus is Fresnosaurus drescheri, which was documented by Welles in 1943. The genus name, Fresnosaurus, is a nod to Fresno County, and the specific name, drescheri, serves as a tribute to Arthur Drescher.

Fresnosaurus was likely a substantial creature, measuring 12 meters (around 39.4 feet) in length. As an elasmosaurid plesiosaur, its diet probably consisted of small bony fish, belemnites, and ammonites. Within the Moreno Formation, several other plesiosaurs have been identified, including Morenosaurus, Aphrosaurus, and Hydrotherosaurus. Samuel Paul Welles described all of these plesiosaurs in 1943, and they all originate from the Moreno Formation.

The Moreno Formation has also yielded various other marine reptiles, such as the mosasaurs Plotosaurus and Plesiotylosaurus, along with the remains of marine turtles.