The Port of Portland is the largest tonnage seaport in New England as well as a major seaport along the east coast of the United States. It is also the second largest oil port on the East Coast, processing 149,616,287 barrels of oil in 2007, mostly through the Portland-Montreal Pipe Line, and the largest foreign inbound transit tonnage port in the United States.Two ferries, The Cat (operated by Bay Ferries) and Casco Bay Lines (operated by the Casco Bay Islands Transit District) serve the Port of Portland and the outlying islands. The Cat serves Yarmouth, Nova Scotia as well.

Portland is the second largest port of call in Maine for cruise ships after Bar Harbor, near Acadia National Park. It is the largest for commercial vessels.The United States Coast Guard patrols the port from stations in Portsmouth, New Hampshire and Boothbay Harbor, Maine.