Champsosaurus is an extinct genus of crocodile-like choristodere reptile, known from the Late Cretaceous and early Paleogene periods of North America and Europe (Campanian–Paleocene).

One was C. ambulator, named from the specimen AMNH 983, a fragmentary skeleton with a partial skull found in the Hell Creek Formation of Montana.

[3] Parks in 1927 named C. albertensis from ROM 806, a partial skeleton lacking the skull, found in the Horseshoe Canyon Formation in Alberta, Canada.

[6] In 1972, Bruce Erickson named the species C. gigas from SMM P71.2.1, a partial skeleton and skull found in the Sentinel Butte Formation, Golden Valley County, North Dakota.

[7] Erickson subsequently in 1981 named the species C. tenuis from SMM P79.14.1, a partial skeleton and skull found in the Bullion Creek Formation, North Dakota.

In 1998 K. Q. Gao and Richard Carr Fox described the species C. lindoei from UALVP 931, a nearly complete skeleton with skull and jaws from the Dinosaur Park Formation in Alberta.

[8] Fossils of Champsosaurus have been found in North America (Alberta, Saskatchewan, Montana, New Mexico, Texas,[9] Colorado, and Wyoming) and Europe (Belgium and France), dating from the Upper Cretaceous to the late Paleocene.

Remains tentatively referred to Champsosaurus are known from the high Canadian Arctic, dating to the Coniacian–Turonian, a time of extreme warmth.

The absence of an otic notch indicates that Champsosaurus lacked a tympanum, and probably had a poor ability to detect airborne sounds.

[15] Champsosaurus belongs to the Neochoristodera, a clade within Choristodera, the members of which are characterised by elongated snouts and expanded temporal arches.

The group first appeared during the Early Cretaceous in Asia, and are suggested to have evolved in the regional absence of aquatic crocodyliformes.

[15] Erickson 1985 suggested that the expanded temporal arches, which likely anchored powerful jaw muscles, and elongated snout allowed Champsosaurus to prey on fish akin to modern gharials, with these adaptions allowing rapid movement of the head and jaws for prey capture.

[21] Erickson 1985 proposed that the position of the nostrils at the front of the snout allowed Champsosaurus to spend large amounts of time at the bottom of water bodies, with the head being angled upwards to allow the snout to act like a snorkel when the animal needed to breathe.

These are hypothesised to be related to breeding behaviour, with the more robust limb bones and fused sacrals of the females allowing them to move themselves onto land to lay eggs.