The Banjo River is the main stream of the Banjo water system in the southern area of Oita Prefecture. The river extends 38km from its source at Mt Haitate to the mouth of Saiki Bay.

There are several theories as to the origin of the river's name. One, is that it's named for a master carpenter named Banjo Toryo Masuemon, who came from Nara and built the Banjo Bridge. Another possible origin is from the name and the shape of Banjo Gane, which was given by the Imperial court to the person in charge of construction. There was also a village called Kawabe-no-Banjo near Saiki castle, which may also be the origin of the name.

In the Edo period, large-scale construction took place to create the four canals of the Saiki domain, which helped the domain's economy.

The Banjo River is one of the major clearwater streams in Kyushu. Its rich ecosystem includes fireflies, kingfishers and crested kingfishers.

The Banjo River festival is held every year in July at the town of Yayoi, and the Cosmos festival takes place in October. These festivals are very familiar to the people who live by the Banjo River.