Relations between Boston, Massachusetts, United States and Halifax, Nova Scotia, Canada date back to the founding of Halifax in 1749, when Boston businesses had an interest in the new settlement for shipping and fish trade. The bond between the two cities became stronger following the Halifax Explosion in 1917, after Boston sent significant aid and disaster relief to Halifax. Boston is the capital of Massachusetts, Boston is the most populous city in the multi-state region New England, and Massachusetts is the most populous state in New England. Likewise, Halifax is the capital of Nova Scotia, Halifax is the most populous city in the multi-province region Atlantic Canada, and Nova Scotia is the most populous province in Atlantic Canada. New England and Atlantic Canada border each other by land and sea along the Canada–United States border. Boston and Halifax are both major port cities on the Atlantic Ocean.

Halifax is the capital and most populous municipality of the Canadian province of Nova Scotia, and the most populous municipality in Atlantic Canada. As of 2022, it is estimated that the population of the Halifax CMA was 480,582, with 348,634 people in its urban area. The regional municipality consists of four former municipalities that were amalgamated in 1996: Halifax, Dartmouth, Bedford, and Halifax County.