In general, sea turtles have a more fusiform body plan than their terrestrial or freshwater counterparts. This tapering at both ends reduces volume and means that sea turtles cannot retract their head and limbs into their shells for protection, unlike many other turtles and tortoises.[8] However, the streamlined body plan reduces friction and drag in the water and allows sea turtles to swim more easily and swiftly.

The leatherback sea turtle is the largest sea turtle, measuring 2–3 m (6–9 ft) in length, 1–1.5 m (3–5 ft) in width, and weighing up to 700 kg (1500 lb). Other sea turtle species are smaller, being mostly 60–120 cm (2–4 ft) long and proportionally narrower.[9]

The skulls of sea turtles have cheek regions that are enclosed in bone.[10][11] Although this condition appears to resemble that found in the earliest known fossil reptiles (anapsids), it is possible it is a more recently evolved trait in sea turtles, placing them outside the anapsids.[12][10]