Lifespan: Unknown to 2/10 of Tenbun 19 (1550)
Father: Ōtomo Yoshiaki
Siblings: Yoshishige (Sōrin), Haruhide (Ōuchi Yoshinaga), Shioichimaru, sister (wife of Ichijō Fusamoto), sister (wife of Kōno Michinobu), sister (wife of Yoshihiro Akimasa), Chikasada
Ōtomo Shioichimaru lived during the Sengoku period. He was a member of the Ōtomo clan, the sengoku daimyō of Bungo Province.
Shioichimaru was born as the third son of Ōtomo Yoshiaki, the twentieth head of the Ōtomo clan. His natural mother was a consort of Yoshiaki while his older brother, Ōtomo Yoshishige (Sōrin) and Ōuchi Yoshinaga were born to a different mother. He had a sister of the same mother.
In 1550, Yoshiaki attempted to transfer headship of the clan to Shioichimaru, whom he cherished, instead of his eldest son, Ōtomo Yoshishige, who was rough and not widely respected. Yoshiaki then plotted with a trusted retainer named Nyūta Chikazane and proceeded to kill retainers who favored Yoshishige, including Kozai Yamato-no-kami and Saitō Nagazane (Shigezane’s father). The succession of killings triggered fear among the retainers in Yoshishige’s camp who then struck back.
On 2/10 of Tenbun 19 (1550), in an event known as the Collapse on the Second Floor, Taguchi Akichika, a member of the kabanshū, or elders of the Ōtomo clan, and leader of the faction supporting Yoshishige, together with Tsukumi Mimasaka-no-kami, attacked Yoshiaki, Shioichimaru, and his mother where they slept on the second floor of the Ōtomo residence. Shioichimaru, his mother, and two of Yoshiaki’s daughters perished in the incident. The attackers met a valorous end at the site of the attack. Meanwhile, Yoshiaki died several days later from injuries sustained in the attack after recording his final wishes concerning the governance of his domain. He was forty-nine years old. Following his demise, retainers led by Bekki Akitsura (later known as Tachibana Dōsetsu) rallied behind Yoshishige to serve as the twenty-first head of the Ōtomo clan.
Yoshishige viewed Nyūta Chikazane, who had supported Shioichimaru, as a ringleader of the plot and, upon his orders, had Akitsura pursue Chikazane. Chikazane fled Higo Province and sought the protection of his father-in-law, Aso Koretoyo, but Koretoyo loathed him for the acts committed and Chikazane was killed. After the attack on his family members, Yoshishige executed those involved, but instability within the clan persisted. In 1553, Yoshishige discovered a plot by Hattori Ukyō-no-suke and other retainers to assassinate him. Yoshishige’s mother was the daughter of the Bōjō family, a kuge. or family of nobility. Alternatively, she may have been the daughter of Ōuchi Yoshioki, in which case the retainers may have sought to remove Yoshishige as a means to reduce the influence of the Ōuchi.