Mauisaurus ("Maui reptile") is a genus of plesiosaur that lived during the Late Cretaceous period around 80 to 69 million years ago in what is now New Zealand. It was the largest plesiosaur, and perhaps the largest marine reptile in New Zealand waters at the time. Mauisaurus haasti is the only known species of the genus. A handful of specimens have been found, although only a few are well preserved and mostly complete. The only other established New Zealand plesiosaur, Tuarangisaurus keyesi, cannot be linked as a direct relative to Mauisaurus.