U.S. Route 91 or U.S. Highway 91 (US 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 Despite the "1" as the last digit in the number, US 91 is no longer a cross-country artery, as it has mostly been replaced by Interstate 15.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.

US 91 begins at Brigham City. The highway was originally routed through Brigham City, but today runs on a southern bypass (locally designated 1100 South). The original routing is now State Route 90. In Brigham City, US 89 becomes concurrent with US 91.

After leaving Brigham City, the highway begins to climb the Wellsville Mountains. The entire grade across these mountains is colloquially called "Sardine Canyon". However, Sardine Canyon is the canyon originally used by the first settlers to mark a trail to Cache Valley, so named as they used the litter from their lunch of canned sardines to mark the trail. A new route was constructed in the 1950s that uses the same mountain pass, Sardine Summit, as the old road, however follows different canyons down from the summit. Despite the modern route of US 91 no longer using Sardine Canyon, the name continues to be used by locals and news media alike in describing the traffic, weather conditions or scenery along the entire grade across the Wellsville Mountains.

US 91 enters the Wellsville Mountains via Box Elder Canyon in a climb to the lake and town of Mantua, which has a reputation as a speed trap. After passing the reservoir, the highway resumes climbing, using Dry Canyon to reach Sardine Summit. The highway descends through Wellsville Canyon and reaches the Cache Valley, where it serves as the major thoroughfare. Logan is the largest city in the valley. At Logan, US 89 splits off to the east towards Bear Lake, while US 91 continues north through the valley to the Idaho state line. Both the Utah and Idaho portions of Cache Valley are part of the Logan metropolitan area.

The Utah section of US 91 is defined at Utah Code Annotated § 72-4-115(1). In addition, the segment south of the SR-142 intersection in Richmond is included in the National Highway System.

US 91 continues serving the Idaho portion of the Cache Valley with Preston as the largest city. The highway passes by the Bear River Massacre Site, a National Historic Landmark, near Preston, and over the geologically significant Red Rock Pass, near Downey.

After leaving Cache Valley, the highway then crosses back to rejoin Interstate 15. At Pocatello, US 91, US 30, and I-15 Business Loop separate from Interstate 15 and run concurrently, passing through a small portion of the Idaho State University campus. US 30 separates from US 91 near Pocatello's city hall at what was once the starting point of one of Idaho's first designated highways, the Yellowstone Park Highway. From this point the highway generally follows the original route of the Yellowstone Park Highway northeastward, until it reaches its terminus at its intersection with US 26, south of Idaho Falls.

At its peak, from 1947 to 1965, U.S. Route 91 extended from the Pacific Ocean at Long Beach, California, to Coutts, Alberta. Though a good portion of the highway fit the US highway numbering grid, overall, US 91's northern terminus was east of US 89, while its southern terminus was west of US 101. North of Riverside, California, US 91 followed a route that would be largely paralleled by modern I-15 and I-215 where these routes did not completely supplant it. There are some parts where US 91/I-15 had divergent paths. The more southerly is from Littlefield, Arizona, to St. George, Utah, where the two routes took different paths through the Arizona Strip. The more northerly and longer is the surviving portion of US 91 from Brigham City to Downey, where Interstate 15 follows an old route of U.S. Route 191.

Utah first submitted a petition to AASHTO to have the US 91 designation truncated in 1971. This petition was approved on June 21, 1971. However, the state did not take action until 1974. Utah officials co-ordinated a second petition with the transportation departments of California, Nevada and Arizona. In this unified petition officials in Utah proposed the number changeover take place in 1974. Nevada requested postponing the deletion of US 91 as Nevada expected to complete the last piece of I-15 in 1975. Nevada officials suggested U.S. Route 40 be similarly truncated, as this highway largely duplicated Interstate 80 in the same states. Despite Nevada's request, the petition was submitted and approved in 1974.

Retracing the path of historic U.S. Route 91 is possible but difficult. In most cities the route of US 91 is still in use as a local street or highway. In many rural areas, I-15 was built on top of the former US 91.

The highway passed through the following states:

• California (until 1974)
• Nevada (until 1974)
• Arizona (10 miles (16 km) in the Arizona Strip, until 1974)
• Utah
• Idaho
• Montana (until 1980)

US 91's original southern terminus was at the intersection of Atlantic Avenue (historically SR 15) and East Pacific Coast Highway (SR 1) in Long Beach. This intersection also marked the historic western terminus of US 6, which proceeded west from that intersection. East of the intersection, US 91 proceeded east along East Pacific Coast Highway to Los Alamitos Circle.

The portions in California were replaced with:

• SR 1 (Pacific Coast Highway from Atlantic Avenue to Los Alamitos Circle)
• SR 19 (Lakewood Boulevard from Los Alamitos Circle to Lincoln Avenue)
• SR 214 (route no longer a state highway – Carson Street and Lincoln Avenue)
• SR 91
• I-215
• I-15

The route of US 91 in Nevada was replaced with I-15 in 1974.

South of Las Vegas, most of the old route is still intact, as it runs parallel and to the east of the freeway from Jean to Sloan. Visitors to the popular Seven Magic Mountains art installation must travel at least a dozen miles on the old route to reach the site.

In Las Vegas, US 91 was better known as Las Vegas Boulevard (State Route 604 after the 1976 renumbering of most Nevada state highways). The intersection of Las Vegas Boulevard and Fremont Street is the historic intersection with U.S. 93 and U.S. 95. Here, U.S. 466 parted ways with US 91 and headed southeast on Fremont Street with southbound US 93 and US 95. At this intersection, US 93 and 95 departed Fremont Street and turned northeast onto Las Vegas Boulevard, heading northbound with US 91. A few blocks further up, at Bonanza Road, US 95 turned west until it hit Rancho Drive, then continued north. US 91/93 stayed aligned with Las Vegas Boulevard, which turned into Salt Lake Highway after departing the city of North Las Vegas.

Until its decertification in 1974, US 91 followed I-15 to exit 112, where it took present-day NV 170 through Riverside to Mesquite, turned right onto NV 144 (Mesquite Boulevard and Sandhill Boulevard), and then followed Hillside Drive into Arizona (where the road became Fairview Avenue).