Onyu Island is inside Saiki Bay and faces the Bungo canal in Oita Prefecture. The island's total area is 5.9km2, and its circumference is 17km around.

It is said that Emperor Jinmu came to this island during the anabasis, and drove the broken bow into the ground. Then, a clear and a fresh stream of water sprang from the ground. Today, there is a place called the Well of the God, where fresh water springs out.

The main industry of this island is fishing, and Onyu Island's chirimen and iriko (two kinds of dried baby sardines) are famous nationwide. The freshly processed sun-dried and cured fish are popular as a souvenir.

Onyu Island is a thriving tourist spot. There are beaches and spots for fishing. Also, there is a 'kangaroo park', with kangaroos that were gifted to the island by its sister city in Australia.

The fire that burns during the Onyu Island Don-do Festival, held every January, is said to have originally come from the fire that was lit to send Emperor Jinmu. The fire-light of this festival is believed to bring good luck. People pray for good health. The fire flames up in a gigantic torch over 10m high, making a spectacular sight.