The Southwest Branch Saint John River is a 62.0-mile-long (99.8 km) river in Maine and Quebec. The branch originates in Little Saint John Lake (46°06′18″N70°15′09″W) on the international boundary between Saint-Zacharie, Quebec and Maine Township 5, Range 20, WELS. The branch forms the Canada – United States border as it flows northeasterly to a confluence with the Little Southwest Branch Saint John River in Maine Township 9, Range 18, WELS. The Southwest Branch flows briefly into Quebec (46°24′58″N70°03′24″W) and then through Maine to its confluence with the Baker Branch Saint John River in Maine Township 9, Range 17, WELS.