Machimosaurus is an extinct genus of machimosaurid crocodyliform from the Late Jurassic (Kimmeridgian and Tithonian) and Early Cretaceous. The type species, Machimosaurus hugii, was found in Switzerland. Other fossils have been found in England, France, Germany, Portugal, Switzerland and Tunisia. Machimosaurus rex is the largest named teleosauroid and thalattosuchian, with an estimated length of up to 7.15 m (23.5 ft) (skull length 155 cm (61 in)). Machimosaurus is the largest known crocodyliform of the Jurassic.

Christian Erich Hermann von Meyer in 1837 named isolated conical, blunt teeth with numerous longitudinal lines from Switzerland, Madrimosaurus hugii. However, in 1838, realising he had misspelled the name, he emended Madrimosaurus to Machimosaurus, from the Greek machimoi, ancient Egyptian troops deployed during the Ptolemaic Dynasty plus the -saurus suffix, literally meaning "pugnacious lizard". The teeth of Machimosaurus, with their rounded, blunt apex and stout morphology make them characteristic and easily identifiable compared to other teleosaurid teeth.

The type species, M. hugii, is known from the Kimmeridgian of Portugal, Spain, Tunisia and Switzerland. Machimosaurus ferox and M. interruptus were previously considered junior synonyms of M. hugii, but have been recently considered possible synonyms of Machimosaurus mosae.

Krebs (1967), considered M. mosae (Lienard, 1876) to be a junior synonym of M. hugii, but is considered a second valid species of the genus based on a nearly complete skeleton found from the late Kimmeridgian of France.

Two species also placed within Machimosaurus are M. bathonicus and M. rigauxi, from the Bathonian of France. However, these are gracile species, lacking the characteristic blunted teeth of Machimosaurus, and are probably referable to Steneosaurus.

Mark Young and his colleagues made a detailed revision of the genus and recognized four species: M. hugii, M. mosae, M. nowackianus from Harrar, Ethiopia, and a new species, Machimosaurus buffetauti. They hypothesized that Machimosaurus may have been analogous to the Pliocene–Holocene genus Crocodylus in having one large-bodied taxon suited to traversing marine barriers and additional, geographically limited taxa across its range.

The fossilized anterior portion of the lower jaw from the Late Jurassic (Oxfordian or Kimmeridgian) of Ethiopia referred to the pliosaur Simolestes nowackianus, is in fact a large species of Machimosaurus.

In 2016, a new species of Machimosaurus from Douiret Formation in Tunisia was described in the journal Cretaceous Research. Named Machimosaurus rex, it was the largest teleosauroid known at the time, estimated to be 9.6 m (31.5 ft) in length (skull length 155 cm (61 in)) based on a partial skeleton. M. rex was also the youngest teleosauroid known at the time. However, more recent estimates put M. hugii along with M. rex at about 6.9–7.15 m (22.6–23.5 ft) long. The discovery of M. rex indicates that teleosauroid crocodylomorphs survived the extinction event at the end of the Late Jurassic, but did not retain the biodiversity seen in the Jurassic. Moreover, an incomplete specimen from the Barremian of Colombia attributed to Teleosauroidea is not only the youngest known teleosauroid, but also the largest at about 9.6 m (31.5 ft) long.