The original forefather of Koizumi Domain was Sadataka KATAGIRI, the younger brother of Katsumoto KATAGIRI who served Hideyoshi TOYOTOMI and earned his fame as one of Shizugatake Shichihon Yari (Shizugatake Seven Spears) during the Battle of Shizugatake in 1583.
小泉藩の藩祖は、豊臣秀吉に仕えて天正11年（1583年）の賤ヶ岳の戦いで賤ヶ岳七本槍の一人として名を馳せた片桐且元の弟・片桐貞隆である。