| Named By: | Othniel Charles Marsh in 1869 |
| Time Period: | Late Cretaceous 86.3 Ma |
| Location: | Angola. Belgium. Jordan. Morocco, Niger. Peru. Sweden. USA. Zaire |
| Size: | Between 3-4 meters long, depending upon the species/individual |
| Diet: | Carnivore/Piscivore |
| Fossil(s): | Skull and post cranial skeletal remains of numerous individuals |
| Classification: | | Chordata | Reptilia | Squamata | Mosasauridae | Halisaurinae | |
| Also known as: | | Baptosaurus | |
Halisaurus is an extinct genus of marine lizard belonging to the mosasaur family. The holotype, consisting of an angular and a basicranium fragment discovered near Hornerstown, New Jersey, already revealed a relatively unique combination of features and prompted a new genus to be described. It was named by Othniel Charles Marsh in 1869 and means "ocean lizard". It was renamed by Marsh to Baptosaurus in 1870, since he believed the name to already be preoccupied by the fish Halosaurus. According to modern rules, a difference of a letter is enough and the substitute name is unneeded, making "Baptosaurus" a junior synonym.
Since its description, more complete remains have been uncovered from fossil deposits throughout the world with particularly complete remains found in Morocco and the United States. The genus remains a key taxon in mosasaur systematics due to its unique set of features and as the most complete representative of its subfamily, the Halisaurinae.
With a length of 3-4 m (9.8-13.1 ft), Halisaurus was comparatively small by mosasaur standards. Though bigger than earlier and more basal mosasaurs, such as Dallasaurus, the sleek Halisaurus would have been dwarfed by many of its contemporaries, such as Tylosaurus and larger species of Clidastes.