Lifespan: Tenshō 13 (1585) to 1/22 of Keian 1 (1648)
Rank: bushō, daimyō
Title: Junior Fifth Rank (Lower), Assistant Master of the Western Capital Office
Domain: Lord of Hitachi-Matsuoka → lord of Dewa-Shinjō
Father: Tozawa Moriyasu
Mother: Daughter of Genzaemon (peasant family in Senboku)
Wife: [Formal] Mamuro Gozen (daughter of Torii Mototada); [Consort] Osako (daughter of Naraoka Mitsunobu); [Consort] Tenōin (daughter of the Kadoya clan)
Children: Masanobu, daughter (formal wife of Tozawa Sadamori), daughter (second wife of Katō Yasuoki)
Adopted Children: Sadamori
Tozawa Masamori served as a bushō and daimyō during the Azuchi-Momoyama and early Edo periods. He was the twentieth head of the Tozawa clan, serving as the lord of the Hitachi-Matsuoka domain and, later, as the first lord of the Dewa-Shinjō domain. He had the common name of Kurōgorō.
In 1585, Masamori was born as the eldest son of Tozawa Moriyasu, a sengoku daimyō based at Kakunodate Castle in Dewa Province. His natural mother was the daughter of Genzaemon, a peasant from Kokomezawa. While engaging in falconry, Moriyasu fell in love with her at first sight. As an illegitimate child of a mother of low social status, he was not eligible to inherit the headship of the clan. Moreover, later, this woman wed a yamabushi, or ascetic hermit, named Tōkōbō so Masamori was raised as the son of a peasant. His father, Moriyasu, died in 1590 and was succeeded by his uncle, Tozawa Mitsumori, who died in 1592 without an heir, so the Tozawa family confronted the prospect of being extinguished. The band of retainers then slayed Tōkōbō and spirited away Masamori to urgently meet with Toyotomi Hideyoshi in Ōsaka and obtain permission for Masamori to inherit the family. On this occasion, Ōmori Gozen (the daughter of Naraoka Mitsukiyo and younger sister of Mitsunobu) who had been the formal wife of Mitsumori accompanied him as his adoptive mother.
In 1600, at the Battle of Sekigahara, Masamori joined the Eastern Army and captured Tōzenji Castle defended by Uesugi Kagekatsu. Owing to this contribution, in 1602, he was awarded a fief of 40,000 koku in the Taga and Ibaraki districts of Hitachi Province and became the lord of the Matsuoka domain. However, accompanying the fall of the Uesugi family, he feared the growing power of the Akita family and was scorned for consistently pursuing passive policies, whereupon his former fief was reduced by 5,000 koku.
In 1614, at the Winter Campaign of the Siege of Ōsaka, he defended Odawara Castle and, in 1615, during the Summer Campaign, he defended Edo Castle, allying with the Tokugawa family. Masamori received as his formal wife the younger sister of Torii Tadamasa, a hereditary senior retainer of the Tokugawa family. He also received the second son of Tadamasa, Tozawa Sadamori, as an heir, deepening ties with the Torii clan and the organs of the bakufu.
In 1622, the Mogami family of the Dewa-Yamagata domain was, upon orders of the Edo bakufu, removed from its position. The former territory of the Mogami was granted to relatives under the leadership of Torii Tadamasa as the next lord of the Yamagata domain while Masamori was transferred to Shinjō and received an increase of 20,000 koku in the Mogami and Murayama districts of Dewa. Masamori became the lord of the Dewa-Shinjō domain with a fief of 60,000 koku and the Tozawa clan was able to return to their homeland in Dewa. Thereafter, he focused his efforts on managing the affairs of the domain.
Masamori died on 1/22 of Keian 1 (1648) at the age of sixty-four. Sadamori died early, so he was succeeded by his son, Tozawa Masanobu.