A plesiosaur is an extinct carnivorous, aquatic reptile with long neck and limbs modified as paddles. The term "plesiosaur" is somewhat loose and may mean a member of the suborder Plesiosauroidea (the "true" plesiosaurs), which includes the elasmosaurs and polycotylids or, alternatively, a member of the larger taxonomic rank of Plesiosauria, which includes the pliosaurs. The plesiosaurs lived during the Jurassic and Cretaceous periods, 190–65 million years ago.

It's been suggested that plesiosaurs may still survive, for example, in the case of the Loch Ness monster. However, more conventional opinion is that they have been long extinct.