Interstate 15 (I-15) is a major Interstate Highway in the western United States. It is the eleventh longestInterstate Highway, following Interstate 20, and is the fourth longest north–south Interstate Highway in the United States. I-15 begins near the Mexican border in San Diego County and stretches north to the Canadian border, passing through the states of California, Nevada, Arizona, Utah, Idaho, and Montana. The interstate serves the major cities of San Diego, Las Vegas, St. George, Salt Lake City, Pocatello and Butte. It also passes close to the urban areas of Orange County, California, Los Angeles County, California, Ogden, Utah, andHelena, Montana.

The stretches of Interstate 15 in Idaho, Utah, and Arizona have been designated as the "Veterans Memorial Highway".

Since the creation of Interstate 15, it has served as a long-haul route for North American commerce. It is now officially chartered for this purpose. From the junction of Interstate 515 in Las Vegas to the Canadian border, I-15 forms part of the CANAMEX Corridor, a High Priority Corridor, as a result of the North American Free Trade Agreement.

Local portions were built to connect the Inland Empire with San Diego in California, facilitate tourism access to Las Vegas, interconnect all of the Metropolitan Statistical Areas in Utah except for Logan, and provide freeway bypasses for Pocatello, Idaho Falls, and Great Falls.

Since the construction of Interstate 15, California, Nevada, and Utah have consistently ranked in the fastest-growing areas of the United States. As a result, the route of I-15 has substantially increased in population and commuter traffic has increased the traffic burden on the freeway. Current population estimates are that more than 75 percent of the population of Utah, 19 percent of the population of California, and more than 70 percent of the population of Nevada live in counties where Interstate 15 is the primary Interstate Highway.

Similarly, in California, I-15 is seeing more commuter traffic due to the growth of the Mojave Desertcommunities of Hesperia and Barstow. In all of these states, I-15 has recently been or is currently in the process of being upgraded to increase capacity. The portions in Arizona, Idaho and Montana have retained their rural, long-haul character. Although Arizona has also grown substantially since the construction of I-15, this highway serves only the isolated corner of northwestern Arizona.

Due to this rapid area growth, the I-15 corridor is the focus of several mass transit projects. The Las Vegas Monorail, FrontRunner commuter rail system and TRAX light rail in Salt Lake City are mass transit lines loosely parallel to I-15 that are now in operation. Between Los Angeles and Las Vegas has long been proposed as amaglev train route; in 2004 the California-Nevada Interstate Maglev project held public meetings on the plan.