Wyoming Highway 138 is a 8.14-mile (13.1 km) east-west (southwest-northeast) Wyoming State Road in central Fremont County.

Wyoming Highway 138 begins its western end at Wyoming Highway 789 east of Hudson. WYO 138 proceeds on a northeasterly direction, and travels through the community (CDP) of Arapahoe. Just northeast of Arapahoe, WYO 138 reaches Wyoming Highway 137 in the unincorporated community of St. Stephens where it ends. Together with WYO 137, Highway 138 provides an alternate to WYO 789 by staying closer to the Popo Agie River.

The entire route is in Fremont County.