The Koizumi Domain (小泉藩 Koizumi-han) was a Japanese domain of the Edo period, with its headquarters within the city limits of present-day Yamato-Kōriyama, Nara. It was ruled for the entirety of its history by the branch of the Katagiri family founded by Katagiri Sadataka, a younger brother of the famed Katagiri Katsumoto. In July 1871, with the abolition of the han system the Koizumi Domain became Koizumi Prefecture, and later it was finally made a part of Nara Prefecture.