Ambulocetus (Latin ambulare "to walk" + cetus "whale") is a genus of early amphibious cetacean[a] from the Kuldana Formation in Pakistan, roughly 48 or 47 million years ago during the Early Eocene (Lutetian). It contains one species, Ambulocetus natans (Latin natans "swimming"), known solely from a near-complete skeleton. Ambulocetus is among the best-studied of Eocene cetaceans, and serves as an instrumental find in the study of cetacean evolution and their transition from land to sea, as it was the first cetacean discovered to preserve a suite of adaptations consistent with an amphibious lifestyle. Ambulocetus is classified in the group Archaeoceti—the ancient forerunners of modern cetaceans whose members span the transition from land to sea—and in the family Ambulocetidae, which includes Himalayacetus and Gandakasia (also from the Eocene of the Indian subcontinent).
Ambulocetus had a narrow, streamlined body, and a long, broad snout, with eyes positioned at the very top of its head. Because of these features, it is hypothesised to have behaved much like a crocodile, waiting near the water's surface to ambush large mammals, using its powerful jaws to clamp onto and drown or thrash prey. Additionally, its ears possessed similar traits to modern cetaceans, which are specialised for hearing and detecting certain frequencies underwater, although it is unclear if Ambulocetus also used these specialised ears for hearing underwater. They may have instead been utilised for bone conduction on land, or perhaps served no function for early cetaceans.
It is thought to have swum much like a modern river otter, tucking in its forelimbs while alternating its hind limbs for propulsion, as well as undulating the torso and tail. It may have had webbed feet, and unlike its modern relatives, lacked a tail fluke. On land, Ambulocetus may have walked much like a sea lion.
Ambulocetus inhabited the Indian subcontinent during the Eocene. The area had a hot climate with tropical rainforests and coastal mangroves, and Ambulocetus may have predominantly inhabited brackish areas such as river mouths. It lived alongside requiem sharks, catfish and various other fishes, turtles, crocodiles, the amphibious hoofed mammal Anthracobune, and the fellow cetaceans Gandakasia, Attockicetus, Nalacetus, and Pakicetus.