Mastodonsaurus (meaning "teat tooth lizard") is an extinct genus of temnospondyl amphibian from the Middle Triassic of Europe. It remains one of the largest amphibians known in the fossil record, and may have exceeded 6 meters (20 feet) in length. Mastodonsaurus belongs to a Triassic group of stereospondyl temnospondyls called Capitosauria, characterized by their large body size, large flattened skulls, and probably predatory aquatic lifestyles analogous to some later crocodilian reptiles.

The gigantic size of Mastodonsaurus made it a top predator in lakes, swamps, river deltas, and other freshwater and brackish aquatic ecosystems. Huge jaws equipped with enlarged fangs and tusks, along with a powerful tail, suggest it was an underwater ambush hunter. Mastodonsaurus would have preyed mainly on fish and on smaller amphibians, but likely took on terrestrial animals that ventured into water as well. Given its heavy body mass and its relatively small limbs, Mastodonsaurus likely could not crawl onto land after reaching larger growth stages.

Like those of many other capitosaurs, the head of Mastodonsaurus was triangular, reaching about 1.5 metres (4.9 ft) in the largest specimens. Narrow grooves on the surface of the skull bones called sulci show it had sensory organs that could detect vibrations and pressure under water, similar to the lateral lines on fish. The large, oval eye sockets are midway along the skull with the nostrils near the tip of the snout. Small ear holes (otic notches) are indented on either side of the back of the skull. The upper surface of the skull bones of Mastodonsaurus bore an intricate pattern of pits and ridges, a feature found in many temnospondyls. The function of this rugged ornamentation is not fully understood. As with other capitosaurs, Mastodonsaurus had a pineal foramen (opening) between the parietal bones behind the orbits on the roof of the skull, which would have contained a light-sensing parietal eye linked to the pineal gland to regulate the circadian sleep-wake cycle and hormone production related to body temperature for a cold-blooded (ectotherm) animal and to reproduction.

The sides of upper jaw are lined with a double row of small conical teeth, while the lower jaw has a single row of similar small teeth. The upper and lower arrangement of small, narrow teeth could function like a trap for small prey when Mastodonsaurus closed its mouth. The tip of the upper jaw has a set of larger teeth. Behind these teeth at the front end of the palate on the underside of the skull are sets of small teeth and multiple pairs of large fangs or tusks (about 8 in all). Two large tusks project up from the end of the lower jaw, fitting through openings on the palate and emerging out from the top of the skull in front of the nostrils when the jaw is closed. The tusk-like teeth on the palate and in the lower jaw could bite and hold bigger prey.

Most of the skeleton of Mastodonsaurus, apart from skulls and jaws, remained poorly known until recently. Both scientific and popular sources continued to describe Mastodonsaurus as having a squat, frog-like body and a short tail from the 19th century into the 20th century, including for the "Labyrinthodon" sculptures by Waterhouse Hawkins at the Crystal Palace outside London in 1854 and in a painting of Mastodonsaurus by the famous Czech paleoartist Zdeněk Burian in 1955. A life-size model put on display for the American Museum of Natural History Hall of Vertebrate Origins in 1996 also restored Mastodonsaurus with a short, broad body and a short tail, and so presumably able to crawl on land.

A site discovered during road construction near the town of Kupferzell in southern Germany in 1977 provided researchers with important new fossils of Mastodonsaurus giganteus that included well preserved skulls and disarticulated bones from all parts of the body. Thousands of individual fossils were recovered during a three-month salvage operation before road work resumed, including, in addition to Mastodonsaurus, remains of the temnospondyl Gerrothorax and the archosaur Batrachotomus, as well as of many fishes. Some of the bones showed evidence of being rolled and transported a long distance. Working from the rich Kupferzell finds, German paleontologist Rainer Schoch published a revised description of Mastodonsaurus in 1999 that revealed a longer body and an estimated longer tail, for a larger, more massive animal with a highly aquatic lifestyle. Although no complete and fully articulated skeleton has been found to date, research since 1999 was incorporated into a composite skeletal reconstruction and a fleshed-out model displayed at the Staatliches Museum für Naturkunde Stuttgart in Germany that give Mastodonsaurus more crocodile-like proportions, with a lengthened tail for swimming, similar to some other capitosaurs.

The exact number of vertebrae in the skeleton is still not known but recent research shows that Mastodonsaurus had about 28 trunk vertebrae and a relatively long tail, revised from the squat body shape and short tail assumed in earlier reconstructions. The total length of the largest individuals is about 4 to 6 metres (13 to 20 ft). Isolated teeth up to 14 cm (6 in) long indicate that old individuals grew even larger.

The marked reduction of the limbs, the strong tail and sensory grooves on the head called sulci show that Mastodonsaurus was an aquatic animal that rarely, if ever, ventured on land. Mastodonsaurus may have been completely unable to leave the water, as large quantities of bones have been found that suggest individuals died en masse when pools dried up during times of drought. It normally inhabited freshwater to brackish swamps, lakes, and river deltas. Fossil skull remains found in marine sediments suggest it also may have entered into saltier environments on occasion.