Keichousaurus is a genus of marine reptile in the pachypleurosaur family which went extinct at the close of the Triassic in the Triassicw-Jurassic extinction event. The name derives from Kweichow (now Guizhou Province) in China where the first fossil specimen was discovered in 1957. They are among the most common sauropterygian fossils recovered and are often found as nearly complete, articulated skeletons, making them popular among collectors. Keichousaurus, and the pachypleurosaur family broadly, are sometimes classified within Nothosauroidea, but are otherwise listed as a separate, more primitive lineage within Sauropterygia.
Keichousaurus, like all sauropterygians, was highly adapted to the aquatic environment. Individuals of this genus ranged up to 2.7 m in length, and had both long necks and long tails, with elongated, five-toed feet. The pointed head and sharp teeth in this genus also indicate that they were fish-eaters. Some recovered specimens feature an especially developed ulna suggesting they may have spent some time on land or in marshes.
In addition fossil evidence suggest also a pair of fossilized pregnant marine reptiles called Keichousaurus hui, show they had a mobile pelvis to give birth to live young rather than laying eggs.