SR 970 was first defined under law to be built and paved by the state of Washington in 1909 as part of the Snoqualmie Pass Road, signed as State Road 7, between Snoqualmie Pass and Blewett Pass. The highway was renamed to the Sunset Highway in 1913 and became State Road 2 during a 1923 restructuring of the state highway system. State Road 2 became concurrent with US 10 from Cle Elum to Teanaway and US 97 from Cle Elum to Virden after the creation of the United States Numbered Highways in 1926. State Road 2 became PSH 2 during the creation of the Primary and secondary state highways in 1937 and US 10 was moved onto US 97 over Blewett Pass in the late 1930s. US 10 was moved to its original route in 1946 after US 2 was extended west from Idaho. After the 1964 highway renumbering, PSH 2 was removed from the highway system and replaced with US 97. US 10 was replaced with SR 10 in 1970 and US 97 was moved east on SR 131 in 1975, constructed two years prior to the realignment. SR 970 was established on the former route of US 97 in 1975 and repairs to the highway were needed immediately, with WSDOT deciding against dropping the designation from the state highway system and turning over the highway to Kittitas County in 1985. No major revisions to the route of SR 970 have occurred since 1975.