A Master-at-Arms (MAA) is a rating responsible for discipline aboard a naval ship.

Royal Navy

The term has been used in the Royal Navy since the time of King Charles I. Originally the Master-at-Arms was a petty officer who looked after personal arms such as swords and firearms, ensuring they were kept in good order and their ammunition was prepared and ready for use in combat. He also instructed other members of the crew in the use of arms. He later also became responsible for discipline. The Ship's Corporals assisted the Master-at-Arms with his duties.

In the modern Royal Navy the MAA is both the ship's police chief and the senior rating, comparable in many respects to the Regimental Sergeant Major in the Army and the Station Warrant Officer in the Royal Air Force. A warrant officer or chief petty officer, the MAA is addressed as "Master"; even if the rating in question is a woman, she is still addresses as "Master" and known as the Master-at-Arms. The MAA is assisted by regulators of the Royal Navy Regulating Branch, of which he is himself a member. He is nicknamed the "jaunty", a corruption of the French gendarme. The non-substantive (trade) badge of an MAA is a crown within a wreath.

Royal Navy MAAs are infamous among young ratings for wreaking verbal havoc on the inattentive recruit who fails to notice their trade badge and addresses them as "Chief" or "Sir" (based on their substantive rank). This is all too common for new entrants to the Navy, as the trade badge is worn on the wrist, and all other naval personnel are addressed according to their epaulette rank insignia only.

United States Navy

In the United States Navy, Master-at-Arms is the rating concerned with law enforcement. They serve as a military police force.