Mauisaurus ("Māui lizard") is a dubious genus of plesiosaur that lived during the Late Cretaceous period in what is now New Zealand.

Numerous specimens have been attributed to this genus in the past, but a 2017 paper restricts Mauisaurus to the lectotype and declares it a nomen dubium.

Mauisaurus haasti was described by Hector in 1874 based on eight specimens and diagnosed by its cervical vertebrae and a humerus with large tuberosities.

The most substantial specimen, 8a (DM R1529), consisted of fragmentary pubes, a partial ilium and hindlimbs, originally misidentified as part of the pectoral girdle.

In 1962 specimen 8a was declared the lectotype of Mauisaurus haasti by Welles who further suggested that M. brachiolatus should be deemed a nomen vanum in an overview of Cretaceous plesiosaurs.

[6][7][8][9] It was later concluded that a hemispherical femoral capitulum, the defining apomorphy of Mauisaurus was also present in members of the Aristonectinae, which referred specimen CM Zfr 115 with its more than 60 neck vertebrae did not belong to.

[10] The presence of femora with strongly hemispherical capitula in more than one aristonectine and also in non-aristonectine elasmosaurids brings Mauisaurus once again into question, with material previously referred to it now being placed in separate clades.