Lifespan: Eikyō 12 (1440) (?) to Eishō 7 (1510) (?)
Other Names: Yoshinaga
Title: Deputy Inspector
Father: Yūki Naotomo
Wife: [Formal] Daughter of Komine Naochika, [Dowager] Daughter of the Ashina clan
Children: Akiyori, Nasu Sukenaga, Gorō
Yūki Masatomo served as a bushō during the late-Muromachi and Sengoku periods. Masatomo was the seventh head of the Shirakawa-Yūki clan. The Shirakawa-Yūki were an illegitimate branch of the Shimōsa-Yūki clan governing the Shirakawa area in the southern portion of Mutsu Province during the Sengoku period.
Masatomo was born as the lineal heir of Yūki Naotomo, the sixth head of the Shirakawa-Yūki clan. Together with his father, Masatomo ushered in the period of peak prosperity for the Yūki clan, wielding authority in the southern portion of Mutsu Province and the northern Kantō. Masatomo wed the daughter of Komine Naochika and a dowager from the Ashina clan.
In 1470, Masatomo allied with the Sōma clan and, in 1474, with the Iwaki. In 1484, he brought on family members of the Ishikawa clan. In 1489, Masatomo allied with the Date, the Ashina, the Oyama, and the Shimōsa-Yūki clans in opposition to the Satake clan.
Masatomo, wed a dowager from the Ashina clan and cherished a son born thereby named Gorō. Even after becoming advanced in age, Masatomo did not endeavor to transfer the headship of the clan to his eldest son, Yūki Akiyori. After maturing, Akiyori felt insecure of his position so, in 1510, with the assistance of Komine Naotsune and members of the Iwaki clan, Akiyori banished Gorō and succeeded to the headship of the clan.
Thereafter, Masatomo came into conflict with the Komine, a powerful branch family, compelling Komine Tomonobu (the eighth head of the Komine clan) to take his own life. Masatomo then incurred an attack by Tomonobu’s father, Komine Naotsune, and fled for the protection of the Nasu clan in connection with a series of events known as the Eishō Conflict. His subsequent whereabouts became unknown. Internal conflicts led to a decline in the influence of the Yūki-Shirakawa clan. Meanwhile, Masatomo is said to have later been murdered by assassins dispatched by Naotsune.
In 1481, to celebrate the prosperity of the Shirakawa-Yūki clan, a renga event was held at the Kashima Shrine below Shirakawa Castle. There are stories regarding the participation by Sōgi, a renga master, in this event.