Leatherback turtles have the most hydrodynamic body design of any sea turtle, with a large, teardrop-shaped body. A large pair of front flippers powers the turtles through the water. Like other sea turtles, the leatherback has flattened fore limbs adapted for swimming in the open ocean. Claws are absent from both pairs of flippers. The leatherback's flippers are the largest in proportion to its body among extant sea turtles. Leatherback's front flippers can grow up to 2.7 m (8.9 ft) in large specimens, the largest flippers (even in comparison to its body) of any sea turtle.
The leatherback has several characteristics that distinguish it from other sea turtles. Its most notable feature is the lack of a bony carapace. Instead of scutes, it has thick, leathery skin with embedded minuscule osteoderms. Seven distinct ridges rise from the carapace, crossing from the cranial to caudal margin of the turtle's back. Leatherbacks are unique among reptiles in that their scales lack β-keratin. The entire turtle's dorsal surface is colored dark grey to black, with a scattering of white blotches and spots. Demonstrating countershading, the turtle's underside is lightly colored. Instead of teeth, the leatherback turtle has points on the tomium of its upper lip, with backwards spines in its throat to help it swallow food and to stop its prey from escaping once caught.
Oesophagus of a leatherback sea turtle showing spines to retain prey D. coriacea adults average 1–1.75 m (3.3–5.7 ft) in curved carapace length (CCL), 1.83–2.2 m (6.0–7.2 ft) in total length, and 250 to 700 kg (550 to 1,540 lb) in weight. In the Caribbean, the mean size of adults was reported at 384 kg (847 lb) in weight and 1.55 m (5.1 ft) in CCL. Similarly, those nesting in French Guiana, weighed an average of 339.3 kg (748 lb) and measured 1.54 m (5.1 ft) in CCL. The largest verified specimen ever found was discovered in the Pakistani beach of Sanspit and measured 213 cm (6.99 ft) in CCL and 650 kg (1,433 lb) in weight, a previous contender, the "Harlech turtle", was purportedly 256.5 cm (8.42 ft) in CCL and 916 kg (2,019 lb) in weight however recent inspection of its remains housed at the National Museum Cardiff have found that its true CCL is around 1.5 m (4.9 ft), casting doubt on the accuracy of the claimed weight, as well. On the other hand, a scientific paper claimed that the species can weigh up to 1,000 kg (2,200 lb) without providing more verifiable detail. The leatherback turtle is scarcely larger than any other sea turtle upon hatching, as they average 61.3 mm (2.41 in) in carapace length and weigh around 46 g (1.6 oz) when freshly hatched.
D. coriacea exhibits several anatomical characteristics believed to be associated with a life in cold waters, including an extensive covering of brown adipose tissue, temperature-independent swimming muscles, countercurrent heat exchangers between the large front flippers and the core body, and an extensive network of countercurrent heat exchangers surrounding the trachea.