northeastern portion of Nova Scotia, Canada. It is separated from the remainder of the province and the Canadian mainland by the 2-mile- (3-kilometre-) wide Strait of Canso (southwest) and is further bounded by the Gulf of St. Lawrence and Cabot Strait (north), the Atlantic Ocean (east and south), and Northumberland Strait (west). The island is 110 miles (175 km) long and up to 75 miles (120 km) wide and has an area of 3,981 square miles (10,311 square km). Administratively it is composed of the counties of Cape Breton, Inverness, Richmond, and Victoria.

Cape Breton Island is mostly hilly and forested and has a highly indented coastline. The island rises in the north to 1,745 feet (532 m) above sea level at the Cape Breton Highlands. The island's centre comprises the 360-square-mile (932-square-kilometre) tidal saltwater Bras d'Or Lake, a popular recreational area.

Originally called Île Royale, when it was a French colony, it later took on the name of its eastern cape—believed to be the first land visited by John Cabot on his 1497–98 voyage and afterward probably named by Basque fishermen from Cap Breton (near Bayonne, Fr.). The island was captured in 1758 by the British, to whom formal cession was made in 1763 in the Treaty of Paris (Paris, Treaty of). It was joined to Nova Scotia but in 1784 became a separate British crown colony. It was rejoined to Nova Scotia in 1820.

Economic activities include coal mining, lumbering, fishing, and summer tourism. Since 1955 the island has been linked to the mainland by a causeway across the Strait of Canso, making it the eastern land terminus of the Trans-Canada Highway. Pop. (2001) 147,500.