Massachusetts Bay is one of the large bays of the Atlantic Ocean that form the distinctive shape of the coastline of the U.S. state of Massachusetts. The bay is enclosed on the north by Cape Ann; at the bay's westernmost point are situated Boston Harbor and the city of Boston. Just south of Massachusetts Bay is Cape Cod Bay.

Sometimes Cape Cod Bay is construed as part of Massachusetts Bay; under this interpretation, the name "Massachusetts Bay" denotes the entire rectangular area of ocean between Cape Ann and Cape Cod.

Massachusetts Bay is itself one corner of a much larger bay, the Gulf of Maine, which reaches from Cape Cod all the way up to Nova Scotia.

The bay gave its name to the Massachusetts Bay Colony, one of the two predecessor colonies of the current state of Massachusetts.