Saiki is a city located in Ōita Prefecture, Japan. As of 31 October 2023, the city had an estimated population of 65,870 in 32921 households, and a population density of 73 persons per km2. The total area of the city is 903.11 km2 (348.69 sq mi), making it the largest city by area in Kyushu.

Ōita Prefecture is a prefecture of Japan located on the island of Kyūshū. Ōita Prefecture has a population of 1,136,245 and has a geographic area of 6,340 km2. Ōita Prefecture borders Fukuoka Prefecture to the northwest, Kumamoto Prefecture to the southwest, and Miyazaki Prefecture to the south.