Nothosaurus are a group of extinct marine reptiles in the superfamily Nothosauroidea. Along with the placodonts, the nothosaurs were the first major group of reptiles to become mostly aquatic (the earlier, smaller mesosaurs were semi-aquatic)  . Nothosaurs were successful throughout the Triassic, and would ultimately give rise to the completely aquatic plesiosaurs, which survived to the end of the Mesozoic era.
- Family Pachypleurosauridae
- Family Nothosauridae