SR 525 was established during the 1964 state highway renumbering and codified in 1970 as the successor to both SSH 1D and SSH 1I. The highway was extended south and east from Highway 99 to the newly-completed I-5 in February 1965, using 164th Street NE until a new freeway could be constructed. The highway was truncated from Anacortes to its present terminus south of Coupeville after SR 20 was extended across the state on the North Cascades Highway in 1973. SR 525 was re-aligned onto a two-lane freeway between SR 99 and the Swamp Creek in the 1980s, extending the route to I-405. The Swamp Creek Interchange itself was completed in November 1984.