Interstate 5 (abbreviated I-5) is the main highway on the West Coast of the United States, paralleling the Pacific Ocean from Washington to California and serving some of the largest cities of the western part of the country, including Seattle, Portland, Sacramento, Los Angeles and San Diego. Its odd number indicates that it is a north-south highway. Its southern terminus is at the international border between the United States and Mexico in the San Diego community of San Ysidro, California. Its northern terminus is at the international border between the United States and Canada at the Peace Arch in Blaine, Washington. I-5 is the only Interstate highway to touch both the Canadian and Mexican borders. At its southern end, I-5 continues into adjacent Tijuana as Mexico Federal Highway 1. On its north side, I-5 continues into Vancouver, Canada as Highway 99.

This highway links to control cities in California (San Diego, Los Angeles, San Francisco, Sacramento, and Redding), Oregon (Medford, Eugene, Salem, and Portland), and Washington (Tacoma, Seattle, Everett, Mount Vernon, Bellingham). Vancouver, British Columbia, Canada, is also a control city on Interstate 5 from the Seattle-Tacoma area to the north terminus at the Canadian border.

Notably, a control city not directly linked by this highway is San Francisco, which is about 80 miles (130 km) west of I-5. To the south, Interstate 580 splits from I-5 towards San Francisco, while, to the north, Interstate 505 cuts south to Interstate 80, which serves that city. That routing, via I-580, I-80 and I-505, was once Interstate 5W.

Along with Interstate 15, Interstate 10, Interstate 8, Interstate 40, and U.S. Route 101, I-5 serves as one of the primary roads that link the Los Angeles/San Diego Metropolitan areas north and east to the rest of the nation.