inlet of the North Atlantic Ocean, extending southward for about 60 miles (100 km) from Cape Ann (Ann, Cape) to Cape Cod (Cod, Cape), Massachusetts, U.S. It includes Nahant, Boston, Plymouth, and Cape Cod bays and Gloucester and Salem harbours. The Atlantic Intracoastal Waterway enters the bay through the Cape Cod Canal and reaches its northernmost point at Boston. Late in 1620, the Pilgrims (Pilgrim Fathers) explored Cape Cod Bay and at Plymouth founded a colony that remained distinct from the Puritan group around Boston (the Massachusetts Bay Colony) until that colony absorbed Plymouth in 1691.

Shipping and industry are concentrated in the Boston area, and numerous cities and towns along the bay serve as tourist, fishing, and yachting centres. Cape Cod is particularly known for its agreeable climate and areas of natural beauty, including the Cape Cod National Seashore (established 1961).