Nautilus, a marine mollusk with a coiled, many-chambered shell. It is also called the chambered, or pearly, nautilus to distinguish it from the argonaut, or paper nautilus, an unrelated mollusk of somewhat similar appearance.

The nautilus is the only surviving representative of a large group of shellfish that lived from 500,000,000 to 200,000,000 years ago. It is distantly related to the ammonite, an extinct mollusk whose fossil remains are common in the United States.

The shell of the nautilus, 4 to 10 inches (10 to 25 cm) across, is prized as an ornament. It is white with brownish stripes. The inside is pearly and divided into chambers, in each of which the nautilus has lived. The animal occupies only the newest, outermost chamber. As the nautilus grows, it secretes a larger chamber and seals off the older one. It also produces a siphuncle—a slender tube that goes back through each chamber and is part of the living animal. The chambers are filled with gas, which gives the nautilus buoyancy and helps it to swim in an upright position. The body of the nautilus resembles that of the octopus and squid, animals of the same biological class as the nautilus. The beaked mouth is surrounded by about 100 suckerless tentacles. A muscular hood partly closes the shell opening.

The nautilus lives on the bottom of the South Pacific and Indian oceans at depths of 200 to 2,000 feet (60 to 600 m). It swims backward, like the squid, by forcing water from the body cavity through a siphon. Its tentacles are used to capture food—chiefly crustaceans and small fish.

The three known species of nautiluses are Nautilus belauensis, N. macromphalus, and N. pompilius of the order Tetrabranchia, class Cephalopoda.