Wyoming Highway 338 begins its southern end in Sheridan at I-90 BUS/US 14 BUS/US 87 BUS (N. Main Street) and exit 20 of Interstate 90, which also carries US 14 and US 87 at this point.

Named Decker Road for the Montana community it reaches, WYO 338 travels north along I-90, paralleling it to the east.

[3] Past 339, WYO 338 curves due east before turning back north on which it will stay to complete its routing.

[3] The highway's southern terminus at I-90 was relocated to a new interchange that opened in August 2017 after a year of construction.

It cost $46.4 million to construct and replaced a trumpet interchange that required traffic to loop under I-90.