Miramichi Bay is an estuary located on the west coast of the Gulf of St. Lawrence in New Brunswick, at the mouth of the Miramichi River. Miramichi Bay is separated into the "inner bay" and the "outer bay", with the division being a line of uninhabited barrier islands which are continually reshaped by ocean storms. The largest of these islands is the uninhabited Portage Island, which was broken in two during a violent storm in the 1950s. The islands provide some protection to the inner bay from ocean storms in the Gulf of St. Lawrence.

The Gulf of St. Lawrence fringes the shores of the provinces of Quebec, New Brunswick, Nova Scotia, Prince Edward Island, Newfoundland and Labrador, in Canada, plus the islands Saint-Pierre and Miquelon, possessions of France, in North America.