Idaho State Highway 21

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.

The road is designated as one of Idaho's scenic byways and provides access to Sawtooth National Recreation Area from Boise and the Treasure Valley. It primarily follows the Boise River and its tributary Mores Creek to the Boise Basin and beyond, and then the upper South Fork of the Payette River and a tributary from Lowman to Banner Creek Summit.