Keelhauling (Dutch kielhalen; "to drag along the keel") is a form of punishment and potential execution once meted out to sailors at sea. The sailor was tied to a line looped beneath the vessel, thrown overboard on one side of the ship, and dragged under the ship's keel, either from one side of the ship to the other, or the length of the ship (from bow to stern).

The common supposition is that keelhauling amounted to a sentence of either death by extreme torture, or minimally a physical trauma likely to cause maim. The hull of the ship was usually covered in barnacles and other marine growth, and thus, keelhauling would typically result in serious lacerations, from which the victim could later suffer infection and scarring. If the victim was dragged slowly, their weight might lower them sufficiently to miss the barnacles, but this method would frequently result in their drowning. There was also a risk of head trauma from colliding against the hull or keel, especially if the ship was in motion.