Interstate 5 (I-5) is the main Interstate Highway on the West Coast of the United States, running largely parallel to the Pacific Ocean coastline from Canada to Mexico (Washington to southernmost California). It serves some of the largest cities on the U.S. West Coast, including Seattle, Portland, Sacramento, Los Angeles, and San Diego, and also links the capital cities of Washington (Olympia), Oregon (Salem), and California (Sacramento) together. The highway's southern terminus is the Mexico – United States border and its northern terminus is the Canada – United States border; it is the only Interstate highway to touch the U.S. borders with both Canada and Mexico. Upon crossing the Mexican border at its southern terminus, Interstate 5 continues to Tijuana, Baja California as Mexico Federal Highway 1. Upon crossing the Canadian border at its northern terminus, it continues to Vancouver, British Columbia as British Columbia Highway 99.

Interstate 5 links to control cities in California (San Diego, Santa Ana, Los Angeles, Oakland, San Francisco, Sacramento, Redding, Mount Shasta, Weed, and Yreka), Oregon (Ashland, Medford, Grants Pass, Roseburg, Eugene, Salem, and Portland), and Washington (Tacoma, Seattle, Everett, Mount Vernon, and Bellingham). Vancouver, British Columbia becomes the highway's control city north of Bellingham; it is signed as "Vancouver B.C." in order to avoid confusion with Vancouver, Washington.