The Celtic Sea is the area of the Atlantic Ocean off the south coast of Ireland bounded to the east by Saint George's Channel; other limits include the Bristol Channel, the English Channel, and the Bay of Biscay, as well as adjacent portions of Wales, Cornwall, and Brittany. The southern and western boundaries are delimited by the continental shelf, which drops away sharply. The Isles of Scilly are an archipelago of small islands in the sea.
The Celtic Sea has a rich fishery with total annual catches of 1.8 million tonnes as of 2007.
Four cetacean species occur frequently in the area: minke whale, bottlenose dolphin, short-beaked common dolphin and harbor porpoise. Formerly, it held an abundance of marine mammals.