Interstate 81 (I-81) is a part of the Interstate Highway System that runs from I-40 at Dandridge, Tennessee, to the Thousand Islands Bridge at Wellesley Island in New York, beyond which the short 2.7-mile (4.3 km) Ontario Highway 137 (Highway 137) links it to Highway 401. In the US state of New York, I-81 extends 183.60 miles (295.48 km) from the Pennsylvania state line southeast of Binghamton to the Canadian border at Wellesley Island northwest of Alexandria Bay. The freeway runs north–south through Central New York, serving the cities of Binghamton, Syracuse, and Watertown. It passes through the Thousand Islands in its final miles and crosses two bridges, both part of the series of bridges known as the Thousand Islands Bridge.

The Thousand Islands International Bridge is an American-maintained international bridge system over the Saint Lawrence River connecting northern New York in the United States with southeastern Ontario in Canada. Constructed in 1937, with additions in 1959, the bridges span the Canada–US border in the middle of the Thousand Islands region. All bridges in the system carry two lanes of traffic, one in each direction, with pedestrian sidewalks.