Halisaurus ("Ocean Lizard") is an extinct genus of marine lizard belonging to the mosasaur family. With a length of 3–4 m (9.8–13.1 ft), it was small compared to most other mosasaurs. It was named by Othniel Charles Marsh in 1869, but renamed Baptosaurus by Marsh in 1870, who thought the name was already in use (preoccupied) by a fish named Halosaurus. According to modern rules, a difference of a letter is enough and the substitute name is unneeded.