either of two genera of cephalopod mollusks: the pearly, or chambered, nautilus (Nautilus), to which the name properly applies; and the paper nautilus (Argonauta), a cosmopolitan genus related to the octopus.

The pearly nautilus has a smooth, coiled external shell about 25 cm (10 inches) in diameter, consisting of about 36 separate chambers, the outermost of which it lives in. The chambers are connected by a tube (the siphuncle) that adjusts the gases in the chambers, allowing the shell to act as a float and maintain neutral buoyancy. Nautilus swims about the ocean using jet propulsion, searching for shrimp or other prey. It uses up to 94 small, suckerless, contractile tentacles for capturing prey. The animals live at depths of 50 to 600 metres (about 160 to 1,970 feet). Nautilus, the last surviving genus of the ancient order Nautiloidea, is important in paleontology for dating the strata in which it appears.

The paper nautilus is usually found near the surface of tropical and subtropical seas feeding on plankton; the females differ from other members of the order Octopoda in that they can secrete a thin, unchambered, coiled shell, formed by large flaps, or membranes, on the dorsal arms, in which eggs are laid and the young hatch. Large shells, which attain a diameter of 30 to 40 cm (12 to 16 inches), are very fragile. The male is only about ¹/₂₀ the size of the female, secretes no shell, and was once thought to be parasitic in the shell of the female. The female resembles the genus Octopus in other features.