The nautilus (from Latin nautilus 'paper nautilus', from Ancient Greek ναυτίλος nautílos 'sailor') is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina.

It comprises nine living species in two genera, the type of which is the genus Nautilus. Though it more specifically refers to species Nautilus pompilius, the name chambered nautilus is also used for any of the Nautilidae. All are protected under CITES Appendix II. Depending on species, adult shell diameter is between 10 and 25 cm (4 and 10 inches).

Nautilidae, both extant and extinct, are characterized by involute or more or less convolute shells that are generally smooth, with compressed or depressed whorl sections, straight to sinuous sutures, and a tubular, generally central siphuncle. Having survived relatively unchanged for hundreds of millions of years, nautiluses represent the only living members of the subclass nautiloidea, and are often considered "living fossils".

The word nautilus is derived from the Greek word ναυτίλος nautílos "sailor", it originally referred to a type of octopus of the genus Argonauta, also known as 'paper nautilus', which were thought to use two of their arms as sails.