Cape Breton Island (French: île du Cap-Breton - formely île Royale, Scottish Gaelic: Eilean Cheap Breatuinn, Míkmaq: Únamakika, simply: Cape Breton) is an island on the Atlantic coast of North America. It likely corresponds to the European word "Breton", referring to Brittany.

Cape Breton Island is part of the province of Nova Scotia, Canada, although physically separated from the Nova Scotia peninsula by the Strait of Canso, it is artificially connected to mainland North America by the Canso Causeway. The island is located east-northeast of the mainland with its northern and western coasts fronting on the Gulf of St. Lawrence; its western coast also forming the eastern limits of the Northumberland Strait. The eastern and southern coasts front the Atlantic Ocean; its eastern coast also forming the western limits of the Cabot Strait. Its landmass slopes upward from south to north, culminating in the highlands of its northern cape. A saltwater estuary, Bras d'Or Lake, dominates the centre of the island.

The island is divided into four of Nova Scotia's eighteen counties: Cape Breton, Inverness, Richmond, and Victoria. Their total population as of the 2001 census numbered 147,454 "Cape Bretoners"; this is approximately 16% of the provincial population. Cape Breton Island has experienced a decline in population of approximately 6.8% since the previous census in 1996. Approximately 72% of the island's population is located in the Cape Breton Regional Municipality (CBRM) which includes all of Cape Breton County and is often referred to as Industrial Cape Breton, given the history of coal mining and steel manufacturing in this area.

The island contains five reserves of the Mi'kmaq Nation, these being: Eskasoni, Membertou, Wagmatcook, We'kopaq/Waycobah, and Potlotek/Chapel Island. Eskasoni is the largest in both population and land area.