Trinacromerum is an extinct genus of sauropterygian reptile belonging to the plesiosaur suborder. It lived during the Late Cretaceous period in what is now Kansas. It was 3 meters (10 feet) long. Its teeth show it fed on small fish.
The long flippers of Trinacromerum enabled it to achieve high swimming speeds. Its physical appearance was described by Richard Ellis as akin to a "four-flippered penguin."