A hulk is a ship that is afloat, but incapable of going to sea. Hulk may be used to describe a ship that has been launched but not completed, an abandoned wreck or shell, or to refer to an old ship that has had its rigging or internal equipment removed, retaining only its buoyant qualities. The word hulk also may be used as a verb: a ship is "hulked" to convert it to a hulk. The verb was also applied to crews of Royal Navy ships in dock, who were sent to the receiving ship for accommodation, or "hulked". Hulks have a variety of uses such as housing, prisons, salvage pontoons, gambling sites, naval training, or cargo storage.

In the days of sail, many hulls served longer as hulks than they did as functional ships. Wooden ships were often hulked when the hull structure became too old and weak to withstand the stresses of sailing.

More recently, ships have been hulked when they become obsolete or when they become uneconomical to operate.