A coast guard or coastguard is a maritime security organization of a particular country. The term embraces wide range of responsibilities in different countries, from being a heavily armed military force with customs and security duties to being a volunteer organization tasked with search and rescue without law enforcement authority. In most countries, a typical coast guard's functions are distinct from those of the navy (a military service) and the transit police (a law enforcement agency), while in certain countries they have similarities to both.

The predecessor of Britain's modern HM Coastguard was established in 1809 as the Waterguard, a department of the HM Customs and Excise authority, which was originally devoted to the prevention of smuggling. At the time, due to high UK taxation on liquors such as brandy, as well as tobacco and other luxuries, smuggling of such cargos from places such as France, Belgium, and Holland was an attractive proposition for criminals. The barrels of brandy and other contraband were landed from the ships on England's beaches at night in small boats and later sold for profit, as later depicted in the Doctor Syn series of books by Russell Thorndike. The Coastguard was also responsible for giving assistance to shipwrecks.

Each Waterguard station was issued with a Manby mortar, which had been invented by Captain George William Manby in 1808. The mortar fired a shot with a line attached from the shore to the wrecked ship and was used for many years. This was the origin of the Coastguard's life saving role. In 1821 a committee of inquiry recommended that responsibility for the Preventative Waterguard be transferred to the Board of Customs. The Treasury agreed and (in a memorandum dated 15 January 1822) directed that the preventative services, which consisted of the Preventative Water Guard, cruisers, and riding officers should be placed under the authority of the Board of Customs and in future should be named the "Coastguard". In 1845 the Coastguard was subordinated to the Admiralty.

In 1829 the first UK Coastguard instructions were published and dealt with discipline and directions for carrying out preventative duties. They also stipulated that, when a ship was wrecked, the Coastguard was responsible for taking all possible action to save lives, to take charge of the vessel and to protect property.

In the United States, the United States Coast Guard was created in 1915 by the merger of two other federal agencies. The first, the United States Revenue Cutter Service, founded in 1790 and known until 1894 as the United States Revenue-Marine, was a maritime customs enforcement agency that also assumed a supporting role to the United States Navy in wartime. The second, the United States Life-Saving Service, was formed in 1848 and consisted of life saving crews stationed at points along the United States East Coast. The Coast Guard later absorbed the United States Lighthouse Service in 1939 and the functions of the Bureau of Navigation and Steamboat Inspection via a two-step process in 1942 and 1946.

Among the responsibilities that may be entrusted to a coast guard service are:

During wartime, some national coast guard organisations might have a role as a naval reserve force with responsibilities in harbor defenses, port security, naval counter-intelligence and coastal patrols.^{[citation needed]}

The coast guard may, varying by jurisdiction, be a branch of a country's military, a law enforcement agency, or a search and rescue body. For example, the United States Coast Guard is a specialized military branch with law enforcement authority, whereas the United Kingdom's His Majesty's Coastguard (HMCG) is a civilian organisation whose primary role is search and rescue. Most coast guards operate ships and aircraft including helicopters and seaplanes that are either owned or leased by the agency in order to fulfil their respective roles.