The Oregon–Walla Walla highway was originally a two-lane road that was the site of hundreds of collisions in the 1960s, prompting the state government to consider new designs.

SR 125 travels parallel to U.S. Route 12 (US 12) on Pine Street for four blocks, crossing under a railroad viaduct in the process, before turning north onto 13th Avenue and passing under US 12.

[6][8] The highway and railroad travel northwest along the floor of Spring Valley, making several turns as they pass through the rural communities of Hadley, Berryman, and Ennis.

[20] By the early 1920s, the Walla Walla–Milton section was part of two signed auto trails: the Theodore Roosevelt International Highway and the California-Banff Bee Line.

[33][34] During the routing debate for Interstate 82 in the late 1960s, the SR 125 and OR 11 corridor was considered as a potential option, but was rejected in favor of the Umatilla Bridge compromise.

[35] The state governments of Oregon and Washington began considering expansion and modernization of the two-lane highway carrying SR 125 and OR 11 in the mid-1960s, following over 400 collisions and 15 deaths in less than a decade.

[38][40] The rejection of several new options presented by the state highway department contributed to the project being delayed into the mid-1970s, along with inflation and the ongoing oil crisis.

[45][46] An abridged version of the project, consisting solely of the four-lane highway expansion without the western bypass, was approved for construction in 1985 and its $4.2 million cost was fully funded using a new state gas tax.

[47][48] Construction began in August 1987 to build the divided highway, which would smooth out curves, include a frontage road, and use a concrete median barrier instead of a center turn lane like the expanded OR 11.

[50] The widening project did not relieve the highway of major collisions, however, due to the merging of traffic from side streets into the fast-moving mainline lanes with a posted speed limit of 55 miles per hour (89 km/h).

[53] The western bypass was ultimately built by the College Place city government in November 2008 as an extension of Myra Road,[54] which was connected to US 12 with a new interchange that opened in 2010.

The spur route travels west for 0.73 miles (1.17 km) along Pine Street from SR 125 at 13th Avenue to a roundabout with Myra Road, where it turns north and terminates at a dogbone interchange with US 12.