U.S. Route 6 (US 6) or U.S. Highway 6 (US 6), also called the Grand Army of the Republic Highway, honoring the American Civil War veterans association, is a main route of the United States Numbered Highway System. While it currently runs east-northeast from Bishop, California, to Provincetown, Massachusetts, the route has been modified several times. The highway's longest-lasting routing, from 1936 to 1964, had its western terminus at Long Beach, California. During this time, US 6 was the longest highway in the country.

The Grand Army of the Republic (GAR) was a fraternal organization composed of veterans of the Union Army, Union Navy, and the Marines who served in the American Civil War. It was founded in 1866 in Decatur, Illinois, and grew to include hundreds of "posts" across the North and West. It was dissolved in 1956 at the death of its last member, Albert Woolson.