Lifespan: 15xx to third month of Tenshō 10 (1582)
Other Names: Genshirō
Lord: Takeda Katsuyori
Father: Yamagata Masakage
Siblings: Masamitsu, Masahisa, Masashige, Nobutsugu, Genpachirō, sister (formal wife of Saigusa Masasada), sister (wife of Aiki Jōrin), sister (formal wife of Yokota Tadamatsu)
Adopted Siblings: Sadamasa, Tarōemon, Saigusa Masasada (?)
Yamagata Masamitsu served as a bushō during the Sengoku and Azuchi-Momoyama periods. He was a retainer of the Kai-Takeda clan and a member of the group of hereditary chief retainers.
Masamitsu was born as the eldest son of Yamagata Masakage, the infamous senior retainer of Takeda Shingen.
His father, Masakage, was the son of Obu Toramasa, but, around 1566, inherited the Yamagata family name. As a close associate of Shingen, Masakage served as an official in the Takeda territory for the issuance of licenses and tax exemptions, deployment orders, governance of shrines and temples, and so forth. He also served as an intermediary with provincial landowners outside the territory such as the Tōyama clan of Mino, the Ashina clan of Mutsu, and the Mikawa-Tokugawa; the Matsuo-Ogasawara, the Muroga, and the Akasu clans of Shinano; and retainers of the Kai-Takeda such as the Saigusa and the Yokota clans.
In 1571, Masakage served as the chamberlain of Ejiri Castle in Suruga Province with responsibility for governing the Tōkai Region. On the fifth month of 1575, he was killed at the Battle of Nagashino and Masamitsu inherited the headship of the clan. During the battle, Masakage’s adopted son, Saigusa Masasada, was also killed in action. Masamitsu retained the support of clans who were aligned with Masakage, such as the Mimura and Haramiishi, but Anayama Nobutada (who held territory in Kawachi) was appointed as the next chamberlain of Ejiri Castle. From that same year, Masamitsu became the chamberlain of Tanaka Castle in Suruga and managed defenses against threats from the direction of Tōtōmi Province.
According to the authenticated biography of Oda Nobunaga known as the Shinchō-kōki, in the third month of 1582, Masamitsu was captured during the Conquest of Kōshū led by Nobunaga and executed.