A Chief Mate (C/M) or Chief Officer, usually also synonymous with the First Mate or First Officer (except on passenger liners, which often carry both), is a licensed member and head of the deck department of a merchant ship. The chief mate is customarily a watchstander and is in charge of the ship's cargo and deck crew. The chief mate is responsible to the Captain for the safety and security of the ship. Responsibilities include the crew's welfare and training in areas such as safety, firefighting, search and rescue.

The Chief Officer, who is the 2IC or second in command of the vessel is often equated, in corporate terms to a senior manager for the operations on board, as the Mate is in charge of a number of departmental functions. In modern cargo vessels, the Mate, holds appointments like, Head of Deck Department, Head of Cargo/ Stowage operations, Head of Safety/ Fire Fighting, Head of on board Security (Ship Security Officer), Head of Environment and Quality amongst other appointments.