U.S. Route 10 Alternate (Washington–Montana)

U.S. Route 10 Alternate was a former U.S. Highway that existed in the states of Washington and Montana. It was designated as an alternate route of U.S. Route 10, providing a more direct route between certain cities while bypassing others. Its primary purpose was to serve local traffic and smaller communities not directly located on the main US 10 corridor. The route's lifespan was relatively short compared to other U.S. Highways, existing from approximately the mid-1920s to the mid-1930s.

The exact routing of U.S. Route 10 Alternate varied slightly over its existence, but generally connected towns and cities that were north of the primary U.S. 10 alignment. In Washington, it generally followed a route through or near towns such as Ellensburg, crossing the Cascade Mountains via a different pass than US 10. In Montana, it served areas of the state north of the main US 10 route.

The U.S. Route 10 Alternate designation was eventually decommissioned. The roads that comprised the former alternate route were either incorporated into other U.S. or state highways, or relinquished to local jurisdiction. Today, no sections remain signed as "U.S. Route 10 Alternate". Remnants of the original roadway are now part of the state and county road networks.

The creation and later decommissioning of U.S. Route 10 Alternate reflects the changing transportation needs of the era. The increasing importance of through traffic and the development of improved highways led to the consolidation of routes and the elimination of alternate designations like this one.