State Route 172 (SR 172) is a 35.01-mile (56.34 km) long state highway serving rural Douglas County in the U.S. state of Washington. The highway travels north from U.S. Route 2 (US 2) near Waterville through Withrow and east through Mansfield before ending at SR 17 at Sims Corner. The highway follows the route of a county road built in the 1930s and signed as Secondary State Highway 10B (SSH 10B) in 1955. SSH 10B became SR 172 during the 1964 highway renumbering and served a daily average of less than 700 vehicles in 2011.

U.S. Route 2 (US 2) is a component of the United States Numbered Highway System that connects the city of Everett in the U.S. state of Washington to the Upper Peninsula of Michigan, with a separate segment that runs from Rouses Point, New York, to Houlton, Maine. Within Washington, the highway travels on a 326-mile-long (525 km) route that connects the western and eastern regions of the state as a part of the state highway system and the National Highway System. US 2 forms parts of two National Scenic Byways, the Stevens Pass Greenway from Monroe to Cashmere and the Coulee Corridor Scenic Byway near Coulee City, and an All-American Road named the International Selkirk Loop within Newport.