Myocastor is a genus of rodent that contains the living nutria (or coypu), as well as several fossil species.

Due to similar cranial morphology, the nutria was once considered a close relative of the Caribbean hutias and placed together with them in the family Capromyidae.

[1] Later, it was more accepted to place it in its own family, the Myocastoridae.

[2] Recent molecular studies place them in the family Echimyidae, in the tribe Myocastorini.

[3][4][5] Kerber et al. (2013) recognize the following species as valid:[6] Other species described but no longer considered valid include Myocastor minor, Myocastor perditus, and Myocastor priscus.