Which turtles are the biggest? #Leatherback! The leatherback sea turtle, the only extant member of the family Dermochelyidae, is the largest of all living turtles and is the fourth largest modern reptile behind three crocodilians. In the #Caribbean, the mean size of adults was reported at 384 kg (850 lb) in weight. The largest ever found weighed 916 kilograms (2,020 lb).