Koyanagawa Munetomo served as a bushō during the Sengoku period. He was a retainer of the Mutsu-Date clan.
The Koyanagawa were members of the Date clan. In 1469, Munetomo was born as the second son of Koyanagawa Morimune. He was a grandson of Date Tokimune, the eleventh head of the Date.
While in his youth, Munetomo went to the capital for training in the sword and military tactics, but then served Ashikaga Yoshiharu, the twelfth shōgun of the Muromachi bakufu. Munetomo also received gold pieces sent by Date Tanemune in Mutsu to support him. After returning to Mutsu, he served Tanemune but, in 1542, after Tanemune was incarcerated by his son and lineal heir, Date Harumune, Munetomo helped rescue Tanemune. During an internal struggle in the Date family known as the Tenbun Conflict, Munetomo sided with Tanemune and, after Tanemune retired, he followed him to Marumori Castle in the Igu District.
According to one account, as a supporter of Tanemune during the Tenbun Conflict, Munetomo sought the assistance of the Ashina, the Sōma, the Tamura, and the Nikaidō clans. In 1565, despite being a member of the Date family, Munetomo was allocated fiefs by the Ashina and Sōma clans for supporting Tanemune.
In the relations between sovereign and subject in the Sengoku period, in some cases, the kunishū, or provincial landowners, residing on or near provincial borders maintained subservient relationships with the lords in more than one province at the same time. Munetomo is noted as an illustration of the complexity of these relationships, but Munetomo was not situated in a border area. In his case, he appears to have received compensation from other lords owing to his role as a go-between for communications between the sengoku daimyō rather than as a result of his residing near a border.
In 1565, after the death of Tanemune, as the ultimate expression of his deep loyalty to his lord, Munetomo martyred himself in front of Tanemune’s grave.