The Mangatainoka River is a river that flows in the Wairarapa region of New Zealand's North Island. Its headwaters are on the eastern side of the Tararua Range to the southwest of the town of Eketahuna, and it roughly parallels the Wairarapa Line railway and State Highway 2 through the northern Wairarapa before meeting the Tiraumea River just before its confluence with the Manawatu River just south of Woodville. The railway's 162 metre bridge across the river between Newman and Hukanui is the longest on the entire line.