Aioi City was founded on October 1, 1942 and was enlarged to its present size on August 1, 1955. The city's name comes from the name of one of the villages that were merged to create the city. (Curiously, the name of the village was pronounced "Au", but used the same kanji characters as the present-day city.) During the Edo period, these villages were part of the Akō han (fiefdom of feudal lords) of Harima Province.