Alexeyisaurus is an extinct genus of possible plesiosaur known from the upper Triassic (lower-middle Norian age) of Wilczek Formation, Wilczek Land, of Franz Josef Land, Russia. It was first named by A. G. Sennikov and M. S. Arkhangelsky in 2010 and the type species is Alexeyisaurus karnoushenkoi. While considered an elasmosaur in the initial publication, it has been described as a "partial, poorly preserved, and undiagnostic sauropterygian skeleton" in subsequent publications.