Lifespan: Bunmei 11 (1479) to 7/13 of Tenbun 16 (1547)
Rank: bushō, sengoku daimyō
Title: Chief of Outer Palace Guards of the Left Division
Father: Yūki Ujihiro
Mother: Daughter of Oda Mochiie
Wife: Daughter of Shigetsuna Utsunomiya
Children: Masanao, Masakatsu, Oyama Takatomo, daughter (wife of Utsunomiya Hisatsuna)
Yūki Masatomo served as a bushō and sengoku daimyō from the late Muromachi to Sengoku period. He was the fifteenth head of the Shimōsa-Yūki clan.
In 1479, Masatomo was born as the son of Yūki Ujihiro, the fourteenth head of the Yūki clan. In 1481, when Masatomo was two years old, his father died. He inherited the headship of the clan at the age of three, but the real power was held by a senior retainer named Tagaya Izumi-no-kami, resulting in tyrannical rule. Meanwhile, Yamakawa Kagesada of the Yamakawa who were members of the Yūki family interfered in the succession, and Kagesada’s son, Yamakawa Motokage, was adopted by the Yūki family.
After attaining adulthood, upon advice of Tagaya Motoyasu, Masatomo murdered Tagaya Izumi-no-kami, regained his authority, and brought order to the Yūki family. He then succeeded in elevating the Yūki to the status of sengoku daimyō. Thereafter, he wed the daughter of Utsunomiya Shigetsuna of Shimotsuke Province. Another daughter of Shigetsuna had wed Ashikaga Takamoto so Masatomo became a brother-in-law of Takamoto and Utsunomiya Tadatsuna, the eldest son and designated heir of Shigetsuna, forming an alliance among them.
Owing to this relationship, during the Eishō Disturbance in 1514, Masatomo, together with his father-in-law, Shigetsuna, sided with Takamoto in the conflict within the Koga kubō (the Kantō branch of the ruling Ashikaga family). Masatomo supported the backing of Takamoto as the next Koga kubō. In the eighth month of 1514, Masatomo allied with Shigetsuna to defeat the combined forces of the Satake and Iwaki clans who came in support of Ashikaga Masauji at the Battle of Takebayashi. Masatomo maintained positive relations with Shigetsuna, forming a powerful alliance between the Shimotsuke-Utsunomiya and the Yūki clans.
In 1516, Shigetsuna died. At the time of his demise, the Shimotsuke-Utsunomiya experienced the peak of their prosperity as the most powerful clan in terms of political and military capabilities in the northern Kantō. Thereafter, Masatomo’s relationship with Shigetsuna’s heir, Utsunomiya Tadatsuna, deteriorated. Tadatsuna feared Masatomo’s superior abilities, and, to eliminate him, forged a secret plan to invade the Yūki territory. In 1523, at the Battle of Kawarada, his lord, Minagawa Muneshige, was killed in action while the Minagawa clan incurred a major blow. As a member of the reinforcements for the Minagawa, Masatomo fought against the Utsunomiya and succeeded in repelling them. This battle marked a definitive break in Masatomo’s relations with the Utsunomiya clan.
In 1526, at the Battle of Saruyama, Masatomo achieved a major victory over the Utsunomiya. Thereafter, he joined with Shigetsuna’s third son, Utsunomiya Okitsuna, who harbored resentment toward Tadatsuna for the reckless invasion and ousted him. By implementing a policy to proactively expand their territory, Masatomo recovered the former territory of the Yūki in Shimotsuke that had been encroached upon by the Utsunomiya from the Muromachi period, expanding his influence into Shimotsuke. Meanwhile, senior retainers including the Tagaya, the Mizunoya, and the Yamakawa asserted their own independence, becoming an issue for subsequent heads of the Yūki clan.
In 1527, Masatomo transferred the headship of the clan to his son, Yūki Masanao, and retired. Thereafter, however, taking advantage of turbulence in the Oyama clan, he backed his son, Yūki Takatomo, to become the head of the Oyama, giving rise to conflicts with the Oda clan among others. Masatomo, who was referred to as the ancestor who rejuvenated the Yūki family, died in 1547.