Mesosuchus ("middle crocodile") is an extinct genus of basal rhynchosaur from early Middle Triassic (early Anisian stage) deposits of Eastern Cape, South Africa. It is known from the holotype SAM 5882, a partial skeleton, and from the paratypes SAM 6046, SAM 6536, SAM 7416 and SAM 7701 from the Aliwal North Euparkeria site. Mesosuchus is quite small, spanning around 30 cm in length. Mesosuchus was discovered and named by D. M. S. Watson in 1912.

Bones of Mesosuchus were first found by David Meredith Seares Watson in 1912 after examining a block of sandstone kept in a private collection of Alfred Brown. This block was found in the middle deposits of the Burgersdorp Formation, in the Cynognathus Assemblage Zone near the town of Aliwal North in the Cape Province of South Africa. The block of sandstone contained many intermingled partial skeletons of several small reptiles, and after careful sorting, Watson considered the unidentified skeletons to belong to a single species, which he named Mesosuchus browni. As the etymology of the name suggests, Watson believed that Mesosuchus was an ancestral crocodile with close affinities to other presumed primitive crocodilians such as Proterosuchus, Erythrosuchus, and Ornithosuchus. However, in 1913, Robert Broom looked more closely at the partial skeletons and immediately determined it to be in fact skeletons of two distinct, though related, species. Broom designated an articulated skeleton with a single external naris and a pair of supposed acrodont premaxillary teeth as the type of Mesosuchus, and the remainder of the specimens were assigned to a new genus and species, Euparkeria capensis.

SAM 5882, the holotype for Mesosuchus, consists of a partial rostrum, palate, braincase, lower jaws, sections of articulated presacral vertebral column, nine articulated caudal vertebrae, portions of scapula and pelvic girdle, and partial forelimb and hindlimbs.

SAM 6046, one of the paratypes of Mesosuchus, consists of an incomplete right maxilla, an articulated series of the last ten presacrals, both sacrals, and first six caudals, partial forelimbs, left and right pelvic girdles, right hind limb, as well as element of left tarsus.

SAM 6536, another paratype, consists of a virtually complete skull with lower jaws, articulated cervical vertebrae and ribs, dorsal vertebrae and ribs, complete left scapulocoracoid and partial right scapula, interclavicle, clavicles, distal end of left humerus, and gastralia.

SAM 7416, another paratype, consists of an articulated vertebral column composed of the last dozen presacrals, both sacrals and at least the first 15 caudal vertebrae, fragments of right forelimb, pelvic girdle, complete right femur, right crus and partial left crus, and right and left tarsi and pedes.

All diagnostic traits of Mesosuchus come from the diagnosis of Mesosuchus browni, as it is the only known species. Most of the autapomorphies used to identify Mesosuchus browni pertain to the skull due to the relatively large amount of skull material known. It was a small rhynchosaurian diapsid with multiple rows of maxillary and dentary teeth, with each row consisting of only a small number of teeth. The two premaxillary teeth are approximately twice the size of the maxillary teeth. The maxillary teeth are inset medially and project below the internal naris. There is also an occlusion between the vomerine teeth and dentary teeth with saddle-shaped vomers that overhang the dorsal side of the premaxillary symphysis. Looking at the spine, the length of the axis neural spine is greater than the length of axis centrum. It also has a craniocaudally narrow neural spine of the third cervical and a prominent midventral groove on the first two caudal centra.

The skull of Mesosuchus has a broadly triangular shape with a wide temporal region that tapers sharply along the orbits, expands abruptly at the prefrontals, and then tapers to the blunt rostrum. There is a large, median external naris located at the front of the rostrum that faces dorsally and cranially. The orbits face laterally and slightly cranially. One feature on the skull that can distinguish the Mesosuchus from all other rhynchosaurs, with possible exception of Howesia, is the presence of a beak-like rostrum, formed primarily by huge premaxillary teeth rather than by tapering, edentulous premaxillae.