A two-decker is a sail warship which carried her guns on two fully-armed decks. Usually additional guns were carried on the upper works (forecastle and quarterdeck) but this was not a continuous battery and so did not count.

Two-deckers ranged all the way from the small 40-gun fourth-rate up to 80- or even 90-gun ships, with the third-rate or "seventy-four" being the archetype.