Sault Ste. Marie is a city in Ontario, Canada. The third-largest city in Northern Ontario after Sudbury and Thunder Bay, it is located on the St. Mary's River on the Canada–US border. To the southwest, across the river, is the United States and the Michigan city of the same name. The two cities are joined by the Sault Ste. Marie International Bridge, which connects Interstate 75 on the Michigan side to Huron Street on the Ontario side. Shipping traffic in the Great Lakes system bypasses the Saint Mary's Rapids via the American Soo Locks, the world's busiest canal in terms of tonnage that passes through it, while smaller recreational and tour boats use the Canadian Sault Ste. Marie Canal.

The St. Marys River, sometimes written St. Mary's River, drains Lake Superior, starting at the end of Whitefish Bay and flowing 74.5 miles (119.9 km) southeast into Lake Huron, with a fall of 23 feet (7.0 m). For its entire length it is an international border, separating Michigan in the United States from Ontario, Canada.