In naval architecture, a poop deck is a deck that constitutes the roof of a cabin built in the aft (rear) part of the superstructure of a ship.

The fantail is an overhang at the extreme rear of the ship, aft of the poop deck and closer to level with the main deck. On a sternwheel steamboat, the fantail supports the large paddlewheel and is typically angled upwards so the shaft of the paddlewheel can sit higher than the main deck. On some steamboats, the extreme end of the fantail supports "monkey" rudders that mimic the operation of the main rudders, in order to provide additional steering control.

The name originates from the French word for stern, la poupe, from Latin puppis. Thus the poop deck is technically called a stern deck, which in sailing ships was usually elevated as the roof of the stern or "after" cabin, also known as the "poop cabin". In sailing ships, with the helmsman at the stern, an elevated position was ideal for both navigation and observation of the crew and sails.

On modern, motorized warships, the ship functions which were once carried out on the poop deck have been moved to the superstructure in the center of the ship, or the island on the starboard side in the case of aircraft carriers.

In nautical parlance, "to be pooped" means to have a wave come over the stern from abaft.