Karatsu is a city located in Saga Prefecture on the island of Kyushu, Japan. Its name, formed from the Japanese word roots 唐 kara, and 津 tsu, signifies its historical importance as an ancient trading port between Japan with China and Korea. The central area of Karatsu, which does not include the former cities and villages of Higashimatsuura District, has a population of 78,386. As of January 1, 2020, the city had an estimated population of 117,663 and a population density of 241 persons per km². The total area is 487.59 km².
On January 1, 2005, the towns of Chinzei, Hamatama, Hizen, Kyūragi, Ōchi, Yobuko and the village of Kitahata were merged into Karatsu.
On January 1, 2006, the village of Nanayama was merged into Karatsu.