At 10,340 feet, South Twin is the second highest summit in the Salmon River Mountains of Idaho, and it sits between White Mountain and North Twin. North Twin, at 10,196 feet, is the fifth highest peak in the Salmon River Mountains. When viewed from Challis, Idaho, South Twin appears to be the highest of the three, but this is only because of the angle it is viewed at. It really is only 98 feet lower than White Mountain. These mountains are on the border of the Frank Church-River of No Return Wilderness as well as the boundary of Custer and Lemhi Counties.

Both North and South Twin Peaks are much more gentle than White Mountain and can easily be climbed together in a day. Because South Twin has a manned fire lookout on its summit, there is a rough road leading all the way to the top. The South Twin lookout is the highest active fire lookout in the Pacific Northwest, and it was built in 1913. The road leading to the lookout is also one of the highest roads in Idaho. The road can also easily be hiked for those not willing to make the drive, and the walk to the top is pleasant. The view from the top is absolutely amazing and includes Borah Peak to the south, the Bighorn Crags in the north, and everything in between...which is probably why this location was chosen for a lookout. In fact, seven major mountain ranges (Pioneer, Boulder, White Cloud, Sawtooth, Salmon River, Beaverhead, Lemhi, and Lost River) are visible from its summit, all of them among Idaho’s highest and wildest. While the road to the top is a pleasant hike, the gentle nature of South Twin makes it easy to hike from Twin Peaks Pass as well. If you'll be climbing in conjunction with North Twin, both peaks can be accessed quite easily from this location. North Twin is an easy hike from the pass and also offers great views. This mountain has three distinct summits, the center one being the highest. Twin Peaks pass offers great camping and a very primitive toilet. It is a great location to set up a base camp and explore the area.