A master-at-arms (US: MA; UK: & Pakistan MAA) may be a naval rating, responsible for law enforcement, regulating duties, security and force protection; an army officer responsible for physical training; or a member of the crew of a merchant ship (usually a passenger vessel) responsible for security and law enforcement. In some navies, a "ship's corporal" is a position—not the rank—of a petty officer who assists the master-at-arms in his various duties.

A master-at-arms (US: MA; UK: & Pakistan MAA) may be a naval rating, responsible for law enforcement, regulating duties, security and force protection; an army officer responsible for physical training; or a member of the crew of a merchant ship (usually a passenger vessel) responsible for security and law enforcement. In some navies, a "ship's corporal" is a position—not the rank—of a petty officer who assists the master-at-arms in his various duties.