Protostegidae is a family of extinct marine turtles that lived during the Mesozoic Era. The family includes some of the largest sea turtles that ever existed. The largest, Archelon, had a head a meter long. While like most sea turtles, they had flattened bodies and had flippers for front appendages, protostegids had minimal shells like leatherback turtles of modern times.

As some of the first marine turtles, the protostegids set the general body plan for future species of sea turtles. Protostegids have a generally depressed turtle body plan, complete with four limbs, a short tail and a large head at the end of a relatively short neck. Like other sea turtles, they possess oar-like front appendages specially evolved for swimming in the open ocean. Similar to the closely related and still-extant Dermochelyidae, protostegids possess extremely reduced carapaces. Some specimens have skeletal protrusions from their ribs almost wrapping around the turtles' bodies in place of a complete shell. Like modern sea turtles, protostegids had sharp beaks. One of the defining characteristics of the members of the family are their almost-disproportionately large heads. Specifically, some specimens of Archelon have been found with heads that were a meter long. In addition, the members of the family had somewhat reduced plastrons as well.