The coxswain (/ˈkɒksən/ KOK-sən or /ˈkɒksweɪn/ KOK-swayn) is the person in charge of a boat, particularly its navigation and steering. The etymology of the word gives a literal meaning of "boat servant" since it comes from cock, referring to the cockboat, a type of ship's boat, and swain, an Old English term derived from the Old Norse sveinn meaning boy or servant. In 1724, a "cockswain" was defined as "An officer of a ship who takes care of the cockboat, barge or shallop, with all its furniture, and is in readiness with his crew to man the boat on all occasions." When the term "cockboat" became obsolete, the title of coxswain as the person in charge of a ship's boat remained.

In rowing, the coxswain sits in either the bow or the stern of the boat (depending on the type of boat) while verbally and physically controlling the boat's steering, speed, timing and fluidity. The primary duty of a coxswain is to ensure the safety of those in the boat. In a race setting, the coxswain is tasked with motivating the crew as well as steering as straight a course as possible to minimize the distance to the finish line. Coxswains are also responsible for knowing proper rowing technique and running drills to improve technique.

A coxswain is the coach in the boat: in addition to following the orders of the team coach, the coxswain is connected to the way the boat feels, what's working, what needs to be changed, and how. A successful coxswain must keep track of the drill, time, pace, words of the coach, feel of the boat, direction of the boat, and safety. During a race, a coxswain is responsible for steering, calling the moves, and responding to the way the other boats are moving. Success depends on the physical and mental strength of the rowers, ability to respond to the environment, and the way in which the coxswain motivates the rowers, not only as individuals but as members of the crew.

In the Royal Navy in the days of sail, the coxswain was a petty officer or chief petty officer who commanded the barge of a captain or admiral. Later the coxswain was the senior deck petty officer or chief petty officer aboard a smaller vessel such as a corvette or submarine, who was responsible for the steering. On smaller vessels, the coxswain assumed the duties that would be performed by the chief boatswain's mate and master-at-arms aboard larger vessels.

In World War I, the term was also used to refer to a chief petty officer who was in charge of steering airships operated by the Royal Naval Air Service.

In World War II pilots of landing craft were referred to as coxswains.

In the Royal Canadian Navy, the appointment of coxswain (or capitaine d'armes in French) is given to the senior non-commissioned officer aboard a ship, the equivalent to a command master chief petty officer in the US Navy. For larger vessels such as a destroyer, frigate or the Harry DeWolf-class ships (AOPVs), a coxswain holds the rank of chief petty officer 1st class (CPO1). For submarines, a coxswain holds the rank of chief petty officer 2nd class (CPO2). For Kingston-class coastal defence vessel, a coxswain usually holds the rank of petty officer 1st class (PO1) or CPO2.

The term was also sometimes used aboard merchant ships for the senior petty officer in charge of the helm. The fictional Israel Hands, for example, was the coxswain of Hispaniola in Treasure Island.