Tachiarai is a town in Mii District, Fukuoka Prefecture, Japan. It lies in the Fukuoka region on the island of Kyushu.
As of 31 January 2024, the town had an estimated population of 16,065 living in 6,283 households, giving a population density of about 700 persons per square kilometre. Its total area is 22.84 square kilometres (8.82 square miles).
These figures make Tachiarai a small, comparatively densely settled municipality within Mii District.