Fragmented skeletal remains of marine turtles occur frequently in archaeological and natural deposits on tropical and subtropical coasts. Identifying these remains to species based on their differential osteomorphology is vital to address questions pertaining to the historical ecology, archaeology, and conservation of marine turtles globally. Although the species‐specific features of extant marine turtle skulls and carapax are relatively well known, the comparative osteomorphology and osteometry of postcranial endoskeletons in closely related species of marine turtles remains unstudied. In this paper, we provide verbal descriptions, line drawings, and photographs of diagnostic morphological criteria for the coracoideum, the humerus, and the femur of two closely related species of Cheloniidae: Chelonia mydas (green turtle) and Caretta caretta (loggerhead), based on observations on modern skeletons. We also present osteometric indices of the humerus and the femur that can be used to distinguish between both species. We comment on the applicability of these criteria on archaeological marine turtle assemblages from the Mediterranean.