Lifespan: Kyōtoku 5 (1456) to 1/9 of Eishō 15 (1518)
Title: Governor of Awa, Assistant Vice-Minister of Popular Affairs, Junior Fourth Rank (Lower) (honorary)
Bakufu: Muromachi – military governor of Bitchū
Father: Hosokawa Noriharu
Mother: Daughter of Tokudaiji Kinari
Siblings: Katsuyuki, Masaharu, Harutomo, Kaneko (formal wife of Nijō Masatsugu), sister (wife of Hino Masasuke), sister
Children: Takakuni, Harukuni, Michimasa (Terumasa), daughter (wife of Hatakeyama Hisanobu)
Hosokawa Masaharu served as a bushō during the late-Muromachi and Sengoku periods. He was the fourth head of the Hosokawa-Yashū family and military governor of Bitchū Province. His older brother, Hosokawa Katsuyuki, and son, Hosokawa Takakuni, were adopted by the Hosokawa-Keichō family and Takakuni later became the head of the Hosokawa-Keichō – the main branch of the Hosokawa clan. His nephew, Hosokawa Tadakata, served as the head of the Hosokawa-Tenkyū family and another nephew, Hosokawa Takamoto, became the military governor of the lower districts of Izumi Province.
Masaharu was born as the son of Hosokawa Noriharu, a shugo daimyō and the third head of the Hosokawa-Yashū during the late Muromachi period. Masaharu received one of the characters in his name from Ashikaga Yoshimasa, the eighth shōgun of the Muromachi bakufu.
The Yashū family, together with the Hosokawa-Awaji and Hosokawa-Tenjiku families, served in the first division of the hōkōshū, the military organ under the direct jurisdiction of the bakufu. Masaharu’s grandfather, Hosokawa Mochiharu, was a close associate of Ashikaga Yoshinori, the sixth shōgun who was assassinated in an event known as the Kakitsu Disturbance. During the attack, Mochiharu lost his left arm. Meanwhile, as can be seen in the cases of Katsuyuki and Takakuni, Masaharu maintained close relations with the Hosokawa Keichō family.
Amidst the foregoing relationships, in 1493, Masaharu suddenly left the camp of Ashikaga Yoshiki (later known as Yoshitane) and rushed to Hosokawa Masamoto, the head of the Hosokawa-Keichō. Thereafter, Masaharu, along with Hosokawa Masakata of the Hosokawa-Tenkyū family, assisted Masamoto. When Asakura Sadakage (who backed Yoshiki) marched south, Masaharu joined Nagashio Bizen-no-kami, a member of the Yasutomi clan, and others to fight against the Tsutsui clan of Yamato who joined forces with Hatakeyama Hisanobu.
In 1507, Masamoto was assassinated in an event known as the Lord Hosokawa Incident. Under traditional theory, in the wake of the killing, Takakuni, as one of three adopted sons of Masamoto, entered directly into a succession struggle to become the next head of the Hosokawa-Keichō family. From 1497, there are traces that Masaharu transferred headship of the clan to Takakuni and he returned to the Yashū family. At the time, Masaharu’s only son was Takakuni. Thereafter, after Takakuni engaged in the succession struggle, Masaharu followed. In 1508, Takakuni ousted Sumimoto and replaced him as the head of the Keichō family. In 1515, after the lineage of his younger brother, Hosokawa Harutomo (the military governor of Bitchū), came to an end, Takakuni appointed Masaharu as the military governor of Bitchū, raising fears of a succession struggle. The issue, however, was resolved with the birth of his son, Hosokawa Harukuni, in the eighth month of 1516.
Around the eighth month of 1517, Masaharu’s physical health declined and he died in 1518 at the age of sixty-three.
For a while, Masaharu was considered to have been the military governor of Izumi but based on more current views that is not the case.