Ceresiosaurus, meaning "Lizard of Ceresio" (Ceresio is the name of the Lake Lugano, in Switzerland), is an extinct genus of aquatic sauropterygian reptile belonging to the nothosaur order. Its fossils have been found in Europe, and was named by Bernhard Peyer in 1931.
Ceresiosaurus lived during the Anisian stage of the middle Triassic.
Olivier Rieppel suggested that Ceresiosaurus is a synonym of Lariosaurus.
Ceresiosaurus was much more elongated than its relatives, reaching 4 metres (13 ft) in length, and had fully developed flippers with no trace of visible toes. It had multiple elongated phalanges, making the flippers much longer than in most other nothosaurs, and more closely resembling those of the later plesiosaurs. Ceresiosaurus also had the shortest skull of any known nothosaur, which further increased its resemblance to plesiosaurs.
Although possessing a long neck and tail, Ceresiosaurus may not have swum by undulating its body. Analysis of the bone structure of the hips and powerful tail suggest that it instead propelled itself through the water much like a penguin. The evidence of pachypleurosaurs in the preserved stomach of Ceresiosaurus remains lend credence to the theory of it being a fast swimmer.