 The original route was certified in 1932; originally routed from Letart Falls (about south of Racine) to east of Letart Falls.The highway was extended to Racine in 1937 along a previously unnumbered road from Letart Falls to Antiquity, and along the former SR 337 from Antiquity to Racine.SR 338 was extended east of Racine along the previous and current alignment of SR 124.