The first section of today's Highway 638 was designated as Highway 561. The route, assigned in the mid-1950s alongside many of the secondary highways in the province, travelled north from Bruce Mines to Ophir, where it turned east to Dunns Valley then north to Skookum Lake. The Department of Highway (DHO), predecessor to the modern
Ministry of Transportation (MTO), assumed the Bruce Mines–Dunn Valley Road on May 9, 1956. On March 29, 1962, Highway 638 was assumed, travelling from Echo Bay to Highway 561. In addition, the portion of Highway 561 between Ophir and Dunns Valley was renumbered as part of the new route; the section north of the Wanamaker Creek
bailey bridge in Dunns Valley to Skookum Lake was decommissioned. This routing remained consistent until mid- to late 1989, when the remaining of Highway 561 was renumbered as part of Highway 638. The segment of Highway 638 between Ophir and Dunns Valley was renumbered as Highway 670 at the same time. The portion of Highway 638 through Bruce Mines was transferred from the province to the town on January 1, 1998.
Highway 17B From 2007 to 2009, Highway 638 extended for an additional westerly from Echo Bay a junction with Highway 17 in
Sault Ste. Marie. The extendion between Echo Bay and Sault Ste. Marie was the former alignment of Highway 17, the route of the
Trans-Canada Highway through the region. It was redesignated as part of Highway 638 when Highway 17 was rerouted along a four-lane
expressway alignment that opened on October 31, 2007. The former alignment of Highway 17 was renumbered, with the portion south of Echo Bay becoming Highway 638A, and the portion northwest of Echo Bay becoming an extension of Highway 638. However, the
Garden River First Nation, citing a 1909
order in council whereby ownership of the highway reverts to the band after realignment, sought to have the route numbered as Highway 17B. The municipal council of
Sault Ste. Marie passed a municipal resolution several weeks later which supported the position of Garden River. In 2008 both Highway 638A and the extension of Highway 638 were renumbered as
Highway 17B. == Future ==