Usuki, city, Ōita ken (prefecture), Kyushu, Japan. The city faces Usuki Bay on the Bungo Channel between the Inland Sea and the Pacific Ocean. An early castle town, Usuki once carried on trade with Portugal. It is now a fishing port and commercial centre; the main industrial activity is brewing. Usuki is perhaps most noted as the site of the former Buddhist Mangetsu Temple, with its ancient rock carvings. Pop. (2005) 43,352; (2010) 41,469.