Aizu (Japanese: 会津) is the old name of part of the modern-day Japanese prefecture of Fukushima, formerly a part of Mutsu province. The principal city of the area is Aizu Wakamatsu.

The eponymous Aizu clan (会津藩; -han), daimyo of the region, with their seat at Aizu Wakamatsu Castle. They had traditionally acted as the bodyguards of the shogun in the feudal Japan of the Edo period. During the Boshin War, Aizu continued to resist even after Shogun Tokugawa Yoshinobu's surrender. The Aizu clan's forces were finally defeated in Aizu Wakamatsu castle in September 1868.

The Byakkotai ("White Tiger Company") was a group of young, predominately teenage, samurai who committed seppuku (a form of ritual suicide) on a hillside overlooking the castle after seeing its defences breached.

The Aizu clan also had close ties to the Shinsengumi.