Aristosuchus (from the Ancient Greek ἄριστος, meaning "bravest, best, noblest", and σουχος, the Ancient Greek corruption of the name of the Egyptian crocodile-headed god Sobek) is a genus of small coelurosaurian dinosaur that lived in the Early Cretaceous period (Barremian stage, sometime between 130 and 123 million years ago) of what is now England, UK. The type and only species is Aristosuchus pusillus, originally referred to the carnosaurian genus Poekilopleuron.

The specimen (BMNH R178) now referred to as Aristosuchus pusillus were discovered by Reverend William D. Fox in deposits from the Isle of Wight, which belonged to the south-western portion of the Wealden Group. BMNH R178 was described in 1876 by Richard Owen, who regarded it as a new species of Poekilopleuron. Poekilopleuron had been named forty years prior by Jacques Amand Eudes-Deslongchamps. Owen, disapproving of how Eudes-Deslongchamps had chosen to spell the genus name, deliberately named his taxon Poikilopleuron pusillus. The species name he gave it, pusillus, originates from the Latin for "very small". Poekilopleuron was at the time believed to be a relative of crocodiles, and Owen saw his taxon in the same way. In 1887, the fossils were revisited by Harry Govier Seeley. Recognising that it differed from Poekilopleuron in key diagnostic features, Seeley opted to give Owen's taxon a new genus name, Aristosuchus. This replacement name comes from the Ancient Greek aristos ("best, superior") and soukhos ("crocodile"). While Seeley loosely supported the idea of Aristosuchus being related to crocodilians, he initially believed that it occupied a taxonomic position between crocodiles and dinosaurs. In his 1901 work Dragons of the Air, Seeley proposed a model of archosaur relationships where "Aristosuchia", a taxon erected previously by Edward Drinker Cope, was separate from saurischians and ornithischians.

Aristosuchus was a bipedal, meat-eating (carnivorous) theropod dinosaur. It is thought to have been about 2 metres (6.6 ft) in length, and is estimated to have weighed about 30 kilograms (66 lb). According to Gregory S. Paul, its weight was estimated at 7 kilograms (15 lb).

The femur of Aristosuchus has a wing-like anterior (front) trochanter and a markedly reduced fourth trochanter.

Since its initial classification as an intermediate between crocodilians and dinosaurs, and its subsequent reclassification into a clade of its own, Aristosuchus has generally been placed in the family Compsognathidae.

A. S. Woodward and C. D. Sherborn, in 1890, regarded Aristosuchus pusillus as being based on the same specimen that Reverend William D. Fox named Calamospondylus oweni in 1866, and many authors followed suit (some regarding C. oweni as a nomen nudum). However, in two papers published in 1999 and 2002, Darren Naish demonstrated that Calamospondylus was based on a different specimen than the Aristosuchus holotype based on letters of correspondence between Richard Owen and Reverend Fox as well as discrepancies in the original description.