Washington State Route 31

State Route 31 (SR 31) is a Washington state highway located entirely in Pend Oreille County. The highway, which is 26.79 miles (43.11 km) long, starts at an intersection with SR 20 in Tiger and travels north to the Canada–US border north of Metaline Falls. At the border, SR 31 becomes British Columbia Highway 6 (BC 6). The route parallels the Pend Oreille River for most of its route and the primary functions of the highway is to serve and connect Tiger, Ione, Metaline and Metaline Falls with British Columbia.

Before 1964, the route from Newport to BC 6 was the northernmost section of Primary State Highway 6, which ran from Spokane to BC 6. Originally created in 1964, SR 31 extended southeast to an intersection with U.S. Route 2 (US 2, formerly US 195) in Newport. The North Cascades Highway (SR 20) was extended to Newport in 1973 and SR 31 from Newport to Tiger became SR 20. SR 31 had one auxiliary route, SR 311, which became SR 211 after SR 20 was extended.