Hydrotherosaurus (meaning "water beast lizard") is an extinct genus of elasmosaurid plesiosaur from the Upper Cretaceous (Maastrichtian stage) of Fresno County, California, USA. It measured up to 13 m in length. The species H. alexandrae was named for its discoverer, Annie Montague Alexander by Samuel Paul Welles.
Species taxanomy provided by GBIF Secretariat (2022). GBIF Backbone Taxonomy. Checklist dataset https://doi.org/10.15468/39omei accessed via GBIF.org on 2023-06-13; License: CC BY 4.0