The only marine turtle to breed in the Galápagos is the Green Sea Turtle (Chelonia mydas agassisi, tortuga verde), which has a hard, dark green to black shell and can weigh up to 150 kg. Unlike the giant tortoise, the female is larger than the male, since she is responsible for swimming while the male mounts and copulates.

Peak mating season is November to January, when both males and females will choose various partners with which to mate. You can often see a mating couple bobbing offshore or swimming near the surface of the water, while other males wait patiently for their turn.

The female then goes ashore to prepare her nest, a large “body pit” with a smaller flask-shaped pit inside where the eggs are laid. She may come ashore as many as eight times at two week intervals, each time depositing seventy to eighty eggs. The small hatchlings emerge after a three-month incubation period, only to be confronted immediately with the reality of their delectability: only about one percent will survive predation by crabs, birds, sharks, or introduced cats, dogs, or donkeys.

Green sea turtles are fairly common throughout the islands, but common nesting spots are Las Bachas (Santa Cruz), Bartolomé, and Gardner Bay (Española).