Interstate 5 in Oregon

Interstate 5 (I-5) in the U.S. state of Oregon is a major Interstate Highway that traverses the state from north to south. It travels to the west of the Cascade Mountains, connecting Portland to Salem, Eugene, Medford, and other major cities in the Willamette Valley and across the northern Siskiyou Mountains. The highway runs 308 miles (496 km) from the California state line near Ashland to the Washington state line in northern Portland, forming the central part of Interstate 5's route between Mexico and Canada.

I-5 was designated in 1957 and replaced U.S. Route 99 (US 99) for most of its length, itself preceded by the Pacific Highway and various wagon roads. The freeway incorporated early bypasses and expressways built for US 99 in the 1950s, including a new freeway route from Portland to Salem, and additional bypasses were built using federal funds. The last segment of I-5, on the Marquam Bridge in Portland, was opened in October 1966 and the whole highway was dedicated later that month. The freeway remains parallel or concurrent to Oregon Route 99 (OR 99) and its spur routes, running along former segments of US 99 that were bypassed by I-5, from Ashland to Portland.

Under Oregon's named route system, all of I-5 is designated as Pacific Highway No. 1. The Salem–Portland section was named the Baldock Freeway until 2022. The freeway also has three signed auxiliary routes that function as spurs and bypasses of major cities: I-105 in Eugene, I-205 in eastern Portland, I-405 in downtown Portland. Two additional auxiliary routes were planned in the early years of the Interstate system, but were shelved after local opposition.