Protostega is an extinct genus of sea turtle containing a single species, Protostega gigas. The species lasted from the Coniacian to the Campanian stages of the Late Cretaceous. Its fossil remains have been found in the Smoky Hill Chalk formation of western Kansas, time-equivalent beds of the Mooreville Chalk Formation of Alabama and Campanian beds of the Rybushka Formation. It is also known from rocks in Canada, with these sediments dating to the middle Campanian due to them being found in the Pembina member. Fossil specimens of this species were first collected in 1871, and named by Edward Drinker Cope in 1872. With a total length of 3.9 metres (13 ft), it is the second-largest sea turtle that ever lived, second only to the giant Archelon, and one of the three largest turtles of all time alongside Archelon and Gigantatypus.