What is the longest international border in the world?

The longest international border in the world is the Canada–United States border.

It stretches for about 8,891 kilometres (5,525 miles), including the boundary between Canada and the contiguous United States and the separate section between Canada and Alaska. The border crosses forests, prairies, rivers, lakes and mountain ranges, and it also passes through the Great Lakes and the Saint Lawrence River.

The boundary was shaped by several agreements, including the Treaty of Paris in 1783, the Jay Treaty, the Treaty of 1818 and the Oregon Treaty. Its modern surveying and maintenance are handled by the International Boundary Commission.

The wording matters: the Canada–U.S. boundary is the world’s longest international border by total length, but the uninterrupted Canada–contiguous-U.S. section is shorter than the Kazakhstan–Russia border. It is also often called the world’s longest undefended border, although both countries maintain customs and law-enforcement controls.