Keichousaurus is a genus of very small nothosauroid marine reptiles. Fossil remains have been found in Mongolia and China, and date to the Norian stage of the Late Triassic period. Keichousaurus is the smallest known fossil marine reptile found to date. The genus Keichousaurus consists of three species: K. hui, K. lusiensis, and K. yuananensis. Size variations and slight morphological differences separate the three species; however, some authors have proposed that such variations represent different stages of growth, rather than separate species.