Thompson Peak is the highest summit in Idaho's Sawtooth Range. Its standard route follows the Fishhook Creek and Alpine Way trails from the Redfish Lake trailhead before leaving the trail to climb the South Couloir to the summit.
From the Redfish Lake trailhead (backpacker parking lot), cross the road and follow the Fishhook Creek Trail about 0.8 miles to a signed junction, then head north through aspens to a ridge top and follow it northwest toward Williams Peak on the Alpine Way trail. After roughly 3.5 miles total from the trailhead, the maintained trail forks; leave it here on an unmarked but well-worn climbers' path that skirts the east flank of Williams Peak, crosses scree and loose gravel, and follows a small stream up to a meadow and an unnamed lake (sometimes called Thompson or Profile Lake) below Thompson Peak. From there, the route climbs to the South Couloir and follows it to the summit, finishing with a short section of Class 3 climbing in the final few feet.
Most parties reverse the ascent route back to the Redfish Lake trailhead.