thumb|398px|link=Plesiosaurus (Greek: πλησιος/plesios, near to + σαυρος/sauros, lizard) was a large marine sauropterygian reptile that lived during the early part of the Jurassic Period, and is known by nearly complete skeletons from the Lias of England, Russia and Germany. It was distinguished by its small head, long and slender neck, broad turtle-like body, a short tail, and two pairs of large, elongated paddles. It lends its name to the order Plesiosauria, of which it is an early, but fairly typical member. P. brachypterygius, P. guilielmiiperatoris and P. tournemirensis were assigned to new genera, Hydrorion, Seeleyosaurus and Occitanosaurus. It lived from 200-65 mya (Jurassic-Cretaceous)
Plesiosaurus was a fairly typical member of its order, and measured around 3 to 5 metres (10 to 16 ft) in total length. The snout was short, but the mouth was able to open very wide, and the jaws were provided with a series of conical teeth in sockets, much like those of the living gavial. The neck was long and slender, but this seems to have been rather stiff, because the vertebrae are nearly flat-ended, which indicates that it could not have been bent in the swan-fashion represented in many old restorations. The other vertebrae are similarly almost flat-ended and firmly united, and there is no sacrum. The ribs are single-headed, and in the middle of the trunk, between the supports of the paired limbs, they meet a dense plastron of abdominal ribs. The short tail was straight and tapered rapidl .The pectoral and pelvic girdles which supported the paired limbs are greatly expanded, the pectoral arch being similar to the corresponding bones of turtles. The limbs were elongated paddles, with five complete digits, although each consists of a very large number of phalanges. Some traces of skin discovered suggest that it was smooth, not scaly.
Plesiosaurus fed on belemnites, fish and other prey. Its U-shaped jaw and sharp teeth would have been like a fish trap. It propelled itself by the paddles, the tail being too short to be of much use. Its neck could have been used as a rudder when navigating during a chase. It is unknown if Plesiosaurus laid eggs on land like sea turtles or gave live birth in the water like sea snakes. The young might have lived in estuaries before going in the open ocean.
- Plesiosaurus is in the video game Zoo Tycoon: Dinosaur Digs
- A Plesiosaurus fights an ichthyosaur in Jules Verne's Journey to the Center of the Earth.
- It appears in Dino Crisis 2 and Dino Stalker.
- It can be created from DNA in Jurassic Park: Park Builder.
- Plesiosaurus was seen as a Holoscape only in Jurassic World. Originally the Plesiosaurus was supposed to be in the fourth film but was cut.
- Plesiosaurus appeared in Jurassic World: The Game.