Lifespan: Tenshō 2 (1574) to Tenshō 17 (1589)
Father: Nihonmatsu Yoshitsugu
Mother: Daughter of Araki Naotsugu
Siblings: Yoshitsuna, Yoshitaka
Wife: [Formal] Daughter of Ōuchi Sadatsuna
Children: Mitsushige (?)
Nihonmatsu Yoshitsuna served as a bushō during the Azuchi-Momoyama period. He was the tenth head of the Nihonmatsu clan and served as the lord of Nihonmatsu Castle in the Adachi district of Mutsu Province.
In 1574, Yoshitsuna was born as the eldest son and designated heir of Nihonmatsu Yoshitsugu, the ninth head of the Nihonmatsu clan. His childhood name was Kuniōmaru. His younger brother was Umeōmaru, later known as Nihonmatsu Yoshitaka.
On 10/8 of Tenshō 13 (1585), when Yoshitsugu paid a visit to Miyamori Castle within the territory of Date Terumune, he abducted Terumune and attempted to take him back to Nihonmatsu Castle. While en route on the Takada Plain, he was chased down by Date Masamune and either died when he and Terumune stabbed one another or was shot and killed along with Terumune and the others in the party. After this event, Kuniōmaru succeeded his father as the tenth head of the Nihonmatsu clan.
Soon after Kuniōmaru succeeded his father, he was attacked at Nihonmatsu Castle by Masamune, but a family member named Araki Nobutsugu led the garrison in defending the castle, while heavy snowfall impeded the siege so the defenders were able to hold out. Meanwhile, on 11/17, at the Battle of Hitotoribashi, Satake Yoshishige defeated the Date forces so the siege was lifted. Nevertheless, the following year, Date Shigezane succeeded in enticing some of those in the castle to collude with the Date, so Kuniōmaru could no longer hold-out against the besieging forces. On 7/16, through the offices of Sōma Yoshitane, Kuniōmaru vacated the castle and withdrew for the protection of the Ashina clan in Aizu. While in Aizu, Kuniōmaru attended his coming-of-age ceremony and adopted the name of Yoshitsuna.
In 1589, Ashina Yoshihiro lost to Masamune at the Battle of Suriagehara, resulting in the decimation of the Ashina clan. In the wake of the defeat, Yoshitsuna and Umeōmaru accompanied Yoshihiro to seek protection from Yoshihiro’s original family, the Satake clan, in Edosaki in Hitachi Province. Yoshitsuna, however, quarreled with Yoshihiro and was assassinated by Numasawa Sanemichi upon orders of Yoshihiro, whereupon Umeōmaru fled from Edosaki to Aizu. Under another theory, he was killed while en route to the Satake territory. He was sixteen years old. As a result, the status of the Nihonmatsu clan as sengoku daimyō came to an end.
According to a genealogy of the Nihonmatsu family, Yoshitsuna had an orphan known as Nihonmatsu Mitsushige, but traces of him are uncertain. Meanwhile, Yohistsuna’s younger brother, Umeōmaru, became a guest of Uesugi Kagekatsu and later served the Mizuno clan under the name of Nihonmatsu Yoshitaka.