State Highway 21 (SH-21), also known as the Ponderosa Pine Scenic Byway, is a state highway in Idaho. It runs from Boise to Stanley, primarily as a two-lane road. With two-thirds of its length in Boise County, it passes by historic Idaho City and the village of Lowman to the western edge of the Sawtooth Mountains, then along their northern boundary to Stanley.

Stanley is a town in the Sawtooth Valley in Custer County, Idaho, United States. The population was 116 at the 2020 census; up from 63 in 2010. The center of population of Idaho in 2000 was located in Stanley.