Which two countries share the longest international border?

Canada and the United States share the world’s longest international border.

The boundary stretches about 8,891 kilometres, including the sections between Canada and the contiguous United States and between Canada and Alaska. It crosses forests, prairies, lakes, rivers, and mountainous terrain, and eight Canadian provinces and territories meet thirteen U.S. states along it.

The border was established in stages through agreements such as the 1783 Treaty of Paris, the Jay Treaty of 1794, the Treaty of 1818, and the Oregon Treaty of 1846. Surveying and maintenance are handled cooperatively by the International Boundary Commission.

It is often described as the world’s longest undefended international border. That phrase means it is not fortified or militarised like many other major boundaries; customs, immigration, and law-enforcement controls still apply. The Canada–U.S. border is longer in total than the continuous Russia–Kazakhstan boundary because its Alaska section is included.