Tangaroasaurus is an extinct genus of squalodontid whale from the Miocene of New Zealand. It contains a single species, Tangaroasaurus kakanuiensis. Similar to Basilosaurus and its close relative Squalodon, it was originally thought to be a species of marine reptile. Parts of the holotype are presumably lost. Its name comes from Tangaroa, the Māori god of the sea, while the suffix -saurus comes from the Latin word for reptile, the group that Tangaroasaurus was originally placed in.

The type fossil was found in a grey clay deposit at All Day Bay and consists of a jaw bearing a few teeth, measuring 5 cm (2.0 in) each. The original describer of the type specimen, William Blaxland Benham, described it as a reptile, either a dinosaur such as Megalosaurus or an late surviving ichthyosaur. The genus was described as an odontocete cetacean in 1979 by R. E. Fordyce.

The status of the genus as a cetacean remains under discussion.

Fossils known from the same geological formation, the All Day Bay formation and Gee Greensand Formation, include an unnamed species of Squalodelphinidae and a species of Prosqualodon.