Nakatsu, city, Ōita ken (prefecture), northern Kyushu, Japan. It lies along the mouth of the Yamakuni River facing the Inland Sea. The city developed around a castle built in 1587 by the Kuroda daimyo family. Industrial development began with the introduction of textile manufacturing in 1896. Nakatsu is now a commercial centre for the porcelain, steel, rice, and silk produced in the surrounding region. The city park, on the old castle site, contains a memorial to Nakatsu’s native son Fukuzawa Yukichi (1835–1901), who founded Keiō Gijuku (now Keiō University) in Tokyo. Pop. (2005) 84,368; (2010) 84,312.