Protostega gigas ('first roof' - 'giant') is an extinct species of marine turtle. It was first collected from the Smoky Hill Chalk of western Kansas in 1871, and named by E.D. Cope (1872). With a length of 3 m (10 ft), it is the second-largest turtle that ever lived, second only to the giant Archelon.
Like the modern leatherback sea turtle, the largest living turtle at 2,70 m (9 ft) long, Protostega's carapace lacked scutes, making it weaker but also lighter. Protostega probably fed on slow ocean creatures such as jellyfish and shellfish.