Trinacromerum is an extinct genus of sauropterygian reptile, a member of the polycotylid plesiosaurs.

Specimens have been discovered in the Late Cretaceous fossil deposits of what is now modern Kansas and Manitoba.

[1] Some fossils are also found in the Southern United States such as in the Mooreville Chalk of Alabama.

[1] The long flippers of Trinacromerum enabled it to achieve high swimming speeds.

[1] Its physical appearance was described by Richard Ellis as akin to a "four-flippered penguin.