The Fencibles (from the word defencible) were British regiments raised in the United Kingdom, Isle of Man and in the colonies for defence against the threat of invasion during the Seven Years' War, the American War of Independence, the French Revolutionary Wars, the Napoleonic Wars and the War of 1812 in the late 18th and early 19th centuries. Usually temporary units, composed of local recruits and commanded by Regular Army officers, they were usually confined to garrison and patrol duties, freeing Regular Army units to perform offensive operations. Most fencible regiments had no liability for overseas service.

They included naval forces known as "River Fencibles", made up of sailors on the Thames and other southern English towns and cities, as well as Sea Fencibles, who, among their other duties, crewed small commercial vessels converted to coastal defence.