Peloneustes (meaning 'mud swimmer') is an extinct genus of sauropterygian reptile belonging to the family Pliosauridae. It is known from the Callovian aged (Middle Jurassic) deposits of the Oxford clay formation of England. With a length of around 3 metres (9.8 ft), it was one of the smallest representatives of the group.
Like its larger relatives, Peloneustes had a short neck and long jaws capable of grabbing large prey. Its streamlined body allowed it to chase fast prey such as belemnites. As it had fewer and blunter teeth than its relatives, it is thought to have mainly fed on hard prey such as ammonites. It is often shown as being in the same waters as Liopleurodon