The flag state of a merchant vessel is the jurisdiction under whose laws the vessel is registered or licensed. A merchant vessel must be registered and can only be registered in one jurisdiction, but may change the register in which it is registered. The flag state has the authority and responsibility to enforce regulations over vessels registered under its flag, including those relating to inspection, certification, and issuance of safety and pollution prevention documents. As a ship operates under the laws of its flag state, these laws are applicable if the ship is involved in an admiralty case.
The term "flag of convenience" describes the business practice of registering a merchant ship in a state other than that of the ship's owners, and flying that state's civil ensign on the ship. Ships are registered under flags of convenience to reduce operating costs, to avoid the regulations of or avoid inspection and scrutiny by the owner's country. Normally the nationality of the ship determines the taxing jurisdiction.
Since the Flag Right Declaration of 1921, it has been recognised that all states—including land-locked countries—have a right to be a flag state. Because of the failure of some flag states to comply with their survey and certification responsibilities, especially flag-of-convenience states that have delegated their task to classification societies, a number of states have since 1982 established Port State Controls of foreign-registered ships entering their jurisdiction.
As at January 2010, Panama was the world's largest flag state, with almost a quarter of the world's ocean-going tonnage registered there. The United States and the United Kingdom had only about 1% each.
Ships must be registered in the ship register of the country whose flag it is flying. Flag registers in many countries are open to ships with foreign owners. Normally, each flag state has only one ship register, but several countries have more than one register:
- Denmark, France and Norway maintain an international register to compete with flags of convenience.
- The Kingdom of the Netherlands allows the different constituent countries to set up their own registeers under the Dutch flag.
- Several territories over which the British Crown holds sovereignty have their own register. Most notably, the Isle of Man has a significant register.
- Hong Kong, part of China, has a separate ship register, the fourth largest in the world,  in addition to China's own ship registry.
Flag state control
Each flag state has set up its own flag state control system:
- in Australia, the Australian Maritime Safety Authority (AMSA) provides flag state control.
- in Canada, Transport Canada is responsible for flag state control under the Canada Shipping Act, 2001.
- in Hong Kong, the Marine Department is responsible for flag state control and enforcement under the Flag State Quality Control Scheme.
- in the United Kingdom, the Maritime and Coastguard Agency (MCA) is responsible for flag state control.
- in the United States, the Coast Guard, under the authority of various federal laws, regulations and international conventions and treaties, the Officer in Charge Marine Inspections is responsible for the inspection of US flag vessels to ensure compliance operating throughout the world.
- in Vanuatu, the Vanuatu Maritime Authority has the responsibility to enforce maritime laws and exercise flag state control.
- Mansell, J.N.K. (2009): Flag State Responsibility: Historical Development and Contemporary Issues, Springer.