Mogami clan
During the winter campaign at the Siege of Osaka he was assigned as castellan of Odawara Castle and during the summer campaign he was castellan of Edo Castle. For these services, Shōgun Tokugawa Ieyasu arranged a marriage between Tozawa Masamori and a daughter of Torii Mototada. Following the attainder of the Mogami clan, he accompanied Torii Tadamasa to Dewa Province and was subsequently assigned a portion of the former Mogami lands, which became Shinjō Domain (60,000 "koku"). The Tozawa clan remained at Shinjō until the Meiji restoration.