Green turtle has been named for its greenish-coloured body fat. It has a dorso-ventrally flattened body, a beaked head at the end of a short neck, and paddle-like arms well-adapted for swimming. The green sea turtle's snout is very short and its beak is unhooked. The horny sheath of the turtle's upper jaw possesses a slightly-denticulated edge while its lower jaw has stronger, more defined denticulation. The dorsal surface of the turtle's head has a single pair of prefrontal shields. Mature C. mydas front appendages have only a single claw, although a second claw is sometimes prominent in young specimens.
Its carapace colour varies with age. Hatchlings of C. mydas, have mostly black carapaces and light-colored plastrons. Carapaces of juveniles are dark brown to olive, while those of mature adults are either entirely brown, spotted or marbled with variegated rays. Underneath, the turtle's plastron is hued yellow. C. mydas limbs are dark-colored and lined with yellow, and are usually marked with a large dark brown spot in the center of each appendage. 
The green sea turtle can grow over one meter in carapace length and weigh about 150 kg.