A boatswain, bo's'n, bos'n, or bosun, also known as a deck boss, or a qualified member of the deck department, is the most senior rate of the deck department and is responsible for the components of a ship's hull. The boatswain supervises the other members of the ship's deck department, and typically is not a watchstander, except on vessels with small crews. Additional duties vary depending upon ship, crew, and circumstances.

The deck department is an organisational team on board naval and merchant ships. The department and its manning requirements, including the responsibilities of each rank are regulated within the STCW Convention, applicable only to the merchant fleets of countries who have ratified it. The department is led by deck officers, who are licensed mariners, and they are commanded overall by the ship's captain. Seafarers in the deck department work a variety of jobs on a ship or vessel, but primarily they will carry out the navigation of a vessel from the bridge. However, they are usually also responsible for supervising and monitoring any maritime cargo on board, as well as ensuring maintenance of the deck and upper hull structure, monitoring the stability of the ship, including loading and discharging ballast water, carrying out mooring operations, and finally anchoring a ship.