Route 338 stretches 241.3 kilometers (149.9 mi) from National Route 5 in Hakodate, Hokkaido south across the Tsugaru Strait by ferry to Ōma, Aomori, around the western, southern, and eastern edges of the Shimokita Peninsula and finally to Oirase.

The highway's brief 1.8 kilometers (1.1 mi) route in Hokkaido ends at the former terminal, which has since been converted into a retail area.

After passing through Sai, the highway enters the western edge of Mutsu and turns east towards the center of the city.

In Mutsu, national routes 279 and 338 briefly meet again, sharing a short concurrency.

[4] National Route 338 was established by the Cabinet of Japan in 1975 between Hakodate and Oirase which was then the town of Shimoda, in Aomori.