The Captain of the Port, in American waters, is a U.S. Coast Guard officer, usually of the rank of captain, who is responsible for Coast Guard law enforcement activities in his area of responsibility. He is usually the commander of a Marine Safety Office in this area. The area may be as small as the waters surrounding a port or encompass a large zone.

A Captain of the Port enforces regulations for the protection and security of vessels, harbors, and waterfront facilities; anchorages; bridges; safety and security zones; and ports and waterways. For example, he may designate restricted areas, such as around nuclear power plants; mandate the use of pilots for navigation into harbor; mandate prior notice of arrival for ships carrying hazardous material; and other safety or security related activities.

In the event of an emergency, the Captain of the Port has the authority to act as necessary without additional authorization and has power over all of the other major Federal agencies in his area of responsibility. However, the Captain of the Port must consider Federal laws and the Code of Federal Regulations in making decisions in non-emergencies.

The designation was first used during World War I and was given to the officer responsible for loading munitions aboard ships.

Other ports world wide, such as Gibraltar, have Captains of the Port, who report to the agency responsible for pilotage.