| Named By: | Edward Drinker Cope in 1871 |
| Time Period: | Late Cretaceous, 83.5 Ma |
| Location: | Canada, USA, and United Kingdom |
| Size: | Larger specimens up to 3.4 meters long (nose to tail) |
| Diet: | Carnivore |
| Fossil(s): | Many specimens |
| Classification: | | Chordata | Reptilia | Testudines | Cryptodira | Protostegidae | |
Protostega ('first roof') is an extinct genus of marine turtle containing a single species, Protostega gigas. Its fossil remains have been found in the Smoky Hill Chalk formation of western Kansas (Hesperornis zone, dated to 83.5 million years ago) and time-equivalent beds of the Mooreville Chalk Formation of Alabama. Fossil specimens of this species were first collected in 1871, and named by Edward Drinker Cope in 1872. With a length of 3 metres (9.8 ft), it is the second-largest sea turtle that ever lived, second only to the giant Archelon, and the third-largest turtle of all time behind Archelon and Stupendemys.