U.S. Route 491 (US 491) is a north–south U.S. Highway serving the Four Corners region of the United States. It was created in 2003 as a renumbering of U.S. Route 666 (US 666). With the US 666 designation, the road was nicknamed the "Devil's Highway" because of the significance of the number 666 to many Christian denominations as the Number of the Beast. This Satanic connotation, combined with a high fatality rate along the New Mexico portion, convinced some people the highway was cursed. The problem was compounded by persistent sign theft. These factors led to two efforts to renumber the highway, first by officials in Arizona, then by those in New Mexico. There have been safety improvement projects in recent years, and fatality rates have subsequently decreased.

The United States Numbered Highway System is an integrated network of roads and highways numbered within a nationwide grid in the contiguous United States. As the designation and numbering of these highways were coordinated among the states, they are sometimes called Federal Highways, but the roadways were built and have always been maintained by state or local governments since their initial designation in 1926.