State Highway 18 (SH 18) is a state highway that runs between the Auckland Northern Motorway (SH 1) on the North Shore and the Northwestern Motorway (SH 16) in Waitakere in Auckland, New Zealand. The Upper Harbour Motorway forms most of its length as of 2016 (the exception being a two-kilometre stretch at the north-eastern end). SH 18 forms the northern part of the Western Ring Route, which continues from Waitakere south-east to Manukau.