As of 2020, which two countries have the longest international border between each other in the world?

The Canada-United States border, officially known as the International Boundary, is the longest international border in the world between two countries. The terrestrial boundary (including boundaries in the Great Lakes, Atlantic and Pacific Ocean is 8,891 kilometers (5,525 mi) long, of which 2,475 kilometers (1,538 mi) is the border of Alaska with British Columbia and Yukon. The agencies responsible for facilitating legal passage through the international boundary are the Canada Border Services Agency (CBSA) and U.S. Customs and Border Protection (CBP).

Eight out of thirteen provinces and territories of Canada and thirteen out of fifty U.S. states are located along this border. Except Prince Edward Island, all Canadian provinces and territories that do not share a border with the U.S. share at least one provincial border with one that does.

Another lengthy international border, and second only to the Canada-US border, is that of Russia-Kazakhstan at 6,846 kilometers (4,253 mi) in length. Meanwhile, South America boasts a long border between Argentina-Chile at an astounding 5,300 kilometers (3,293 mi). The Mongolia-China boundary is the world's fourth longest international border between two countries at 4,677 kilometers (2,906 mi) long.