By 1947, Iowa Highway 44 had been created along the same route Iowa 15 now occupies. In 1969, then-Iowa Highway 44 was redesignated as Iowa Highway 15 to match Minnesota State Highway 15 at the state line.
Iowa Highway 44 and Iowa 15 were originally separate roads but by 1947 they shared a common route. In 1969, this section of road was renumbered as Iowa Highway 15 to match Minnesota State Highway 15 at the state border.