Pantosaurus ("all lizard") is an extinct genus of plesiosaur from the Late Jurassic (Oxfordian) of what is now Wyoming.

[1][2] The species Muraenosaurus reedii is in fact a junior synonym of Pantosaurus.

[3] The holotype YPM 543 is a partial articulated skeleton, partially prepared to yield a distal humerus, four articulated carpals, a fragment of the coracoid, and several isolated cervical vertebrae from the Upper Member of the Sundance Formation.

[4] Pantosaurus possesses between 35 and 40 cervical vertebrae, which are very similar in proportion and morphology to those of Muraenosaurus leedsii from the Oxford Clay Formation (Callovian, Middle Jurassic) of England.

The forelimb of Pantosaurus however can be differentiated from that of Muraenosaurus, such as the relatively large size of the radius and the corresponding humerus-radius articulation.