Idaho State Highway 39 begins at an interchange with Interstate 86 next to the American Falls Airport. The highway proceeds west, bypassing the community of American Falls. The road travels along the American Falls Dam, before bending northward, and traveling parallel to American Falls Reservoir. The highway continues, bending northeast, before bending back northward, and passing through the community of Aberdeen. The roadway continues north, before traveling northeast for a short distance, and turning north again. The route continues north, before bending eastward and proceeding. The highway proceeds eastward for several miles, before bending northeastward. The highway continues northeastward, and, after a long distance, passes the Rockford Municipal Airport. The road bends eastward, passing through the community of Moreland. The highway continues eastward to its eastern terminus, an intersection with U.S. Route 26.