When the Sekigahara War broke out in September of the same year, Masamune joined the Eastern Army led by Ieyasu; when the troops of Uesugi clan led by Kanetsugu NAOE attacked the Yamagata-jo Castle where Mogami family resides, Masamune sent reinforcements led by his deputy, Masakage RUSU for Mogami clan (the Battle of Hasedo-jo Castle).
同年9月、関ヶ原の戦いになると、家康ら東軍に属した政宗は、上杉氏の将直江兼続率いる軍が最上氏居城山形城を攻撃すると、留守政景を名代として最上に援軍を派遣した（長谷堂城の戦い）。