Remains of a large sea turtle, Terlinguachelys fischbecki n. gen. and sp., were recovered from paralic deposits of the Upper Cretaceous (Campanian) Aguja Formation in Big Bend National Park, Texas. T. fischbecki is a primitive protostegid that retains a constricted humerus, well ossified plastron and costals, prominent retroarticular process on the lower jaw, and long slender femora; however, it has some features, such as a prominent tubercle at the base of the scapular acromion process, found elsewhere only in derived leatherback sea turtles. The unique combination of primitive and derived traits in T. fischbecki illustrates further diversity among Cretaceous sea turtles and another case of parallelism common in sea turtle phylogeny.