U.S. Route 91 or U.S. Highway 91 is a 172.7-mile-long (277.9 km) north–south United States highway running from Brigham City, Utah, to Idaho Falls, Idaho, in the U.S. states of Idaho and Utah. Despite the "1" as the last digit in the number, US 91 is no longer a cross-country artery, as it has mostly been replaced by Interstate 15. The highway currently serves to connect the communities of the Cache Valley to I-15 and beyond. Prior to the mid-1970s, US 91 was an international commerce route from Long Beach, California, to the Canada–US border north of Sweetgrass, Montana. US 91 was routed on the main streets of most of the communities it served, including Las Vegas Boulevard in Las Vegas and State Street in Salt Lake City. From Los Angeles to Salt Lake, the route was built along the corridor of the Arrowhead Trail. A portion of the highway's former route in California is currently State Route 91.

Brigham City is a city in Box Elder County, Utah, United States. The population was 19,650 at the 2020 census, up from the 2010 figure of 17,899. It is the county seat of Box Elder County. It lies on the western slope of the Wellsville Mountains, a branch of the Wasatch Range at the western terminus of Box Elder Canyon. It is near the former headquarters of ATK Thiokol, now Northrop Grumman, the company that created the solid rocket boosters for the Space Shuttle.