Kōsokabe Hideyoshi served as a bushō during the Sengoku period.
Hideyoshi was the son and designated heir of Kōsokabe Chikahide, but, in 1526, when Chikahide attacked the Aki clan in the Aki District of Tosa, Hideyoshi was killed in route to a defeat by Chikahide. Having lost his heir, Chikahide adopted his younger brother, Kōsokabe Hidemichi, to become his heir. Later,
Later, fearing the expansion of power wielded by Chōsokabe Kunichika, he adopted his third son, creating a conflict with Hidemichi. Hidemichi was fiercely opposed to joining forces with Kunichika. In the end, in 1556, Chikahide assassinated Hidemichi and received Kunichika’s third son as Kōsokabe Chikayasu to become his heir.
The loss of Hideyoshi in battle was a factor in the ultimate downfall of the Kōsokabe clan.