The Piako River is a lowland river system that drains into the Firth of Thames on the North Island of New Zealand. Together with the Waihou River, it is one of the two main rivers systems which drains the Hauraki Plains. It is the dominant river system in the Matamata-Piako District, and on its journey it passes through the towns of Morrinsville and Ngatea.
The Piako River has an annual mean flow of 17 m3 s−1 (at Paeroa-Tahuna Road), with a total catchment area of approximately 1,440 km2. The river system is approximately 100 km in length, aligned in a north-south direction and occupies much of the central and western Hauraki Plains (or more broadly, the Thames Valley), extending from Hinuera in the south before discharging into the Firth of Thames in the north, five kilometres west of Thames.