Sumimoto was a son of Hosokawa Yoshiharu and an adopted son of Hosokawa Masamoto. After Masamoto was killed, Sumimoto fled from his brother Sumiyuki and took up refuge with Rokkaku Takayori of Ômi. A powerful Hosokawa vassal, Miyoshi Nagateru, raised troops in Settsu and destroyed Sumiyuki in the young Sumimoto's name. Though practically still a child, Sumimoto was named kanrei and inherited all of the Hosokawa's holdings on Shikoku. Masamoto's 3rd son Takakuni and the shôgun he deposed, Ashikaga Yoshitane, had fled to Suo province and garnered the support of Ôuchi Yoshioki, who raised an army and marched east. Sumimoto and his followers had intended to face Yoshioki in Settsu, but fled to Shikoku when they caught a glimpse of the large Ôuchi army (1508). Sumimoto returned to the Kyoto area in 1511 but was defeated by the Ôuchi at Funaokayama and fled once more. In 1519, after Ôuchi had left the capital, Sumimoto attempted another return to Kyoto, but was defeated by Hosokawa Takakuni and the Rokkaku. He withdrew back to Shikoku and died the next year.
- Initial text from Sengoku Biographical Dictionary (Samurai-Archives.com) FWSeal & CEWest, 2005