State Highway 21 (SH 21) is a short 6.7 kilometre stretch of highway in the North Island of New Zealand. It links State Highway 1, Waikato Expressway at Tamahere and State Highway 3 at Ōhaupō. Its main destination is Hamilton Airport and Mystery Creek, where the National Agricultural Fieldays are held. SH 21 was gazetted as a brand new state highway designation in 1997.