SR 107 begins at an intersection with US 101 in rural Grays Harbor County between Cosmopolis and Raymond.

SR 107 turns north in South Montesano and crosses the Chehalis River into the city of Montesano before the designation ends at a diamond interchange with US 12,[3] while the roadway continues north as Main Street into the city.

[9][10] SR 107 was established during the 1964 highway renumbering to replace the PSH 9 branch and was codified in 1970.

[14][15] The 731-foot (223 m) Chehalis River bridge was rehabilitated by the state government from 2019 to 2021 at a cost of $25.2 million, funded by the Connecting Washington package passed in 2015.

[16] The project included repainting the bridge's steel truss to prevent corrosion, replacing the bridge deck, and replacing the wooden trestle on the south approach with concrete and steel pilings.