The river Loimijoki is a river in Finland and the longest tributary of the river Kokemäenjoki. The river originates at the lake Pyhäjärvi in Tammela and joins the river Kokemäenjoki in Huittinen. There is a 54 meter difference in the elevation of the source and the end of the river and it is 114 km long. The river drains a catchment area of 3140 km². The river has several dams at Forssa, Jokioinen and Loimaa.

The Loimijoki river runs through the most fertile lands of Finland making its water relatively muddy. The river was badly polluted by sewage of industry and population centers along the river, but since 1980s the river has been starting to recover and currently the river sees also recreational use like fishing and water sports.