Lifespan: Daiei 2 (1522) to 8/1 of Tenshō 9 (1581)
Other Names: Akiuji, Mutō Yoshimasu
Name Changes: Shinkurō → Haruchika → Yoshimasu → 証江庵
Rank: bushō, daimyō
Clan: Daihōji (descended from the Mutō clan of the Fujiwara lineage)
Lord: Ashikaga Yoshiharu → Ashikaga Yoshiteru → Uesugi Kenshin
Father: Daihōji Kurō
Children: Yoshiuji, Yoshioki
Daihōji Yoshimasu served as a bushō and daimyō during the Sengoku and Azuchi-Momoyama periods. He was the sixteenth head of the Daihōji clan and served as the bettō, or steward of the Three Sacred Mountains of Dewa.
Genealogical charts relevant to Yoshimasu give rise to various theories regarding his origins. Yoshimasu was the son of Daihōji Kurō and a cousin of Daihōji Harutoki. He grandfather was Daihōji Masauji. Yoshimasu was separately known as Haruchika, with the character of “haru” from the name of his older brother, Harutoki, which character originated from Ashikaga Yoshiharu, the twelfth shōgun of the Muromachi bakufu. The character of “yoshi” appears to have come from Yoshiharu or his son, Ashikaga Yoshiteru.
After the death of Harutoki, through the support of a retainer named Tosabayashi Zentō, Yoshimasu became the successor to Harutoki, but, lacking the capability to govern the clan, internal conflicts persisted in his territory. Yoshimasu survived owing to support from Honjō Shigenaga of Echigo Province (with whom he had ongoing relations) and the Onodera clan in return for having sheltered Onodera Kagemichi when Kagemichi temporarily fled to Shōnai following the murder of his eldest son.
In 1563, Yoshimasu defeated Sasaki Sadatsuna and captured Sakenobe Hidetsuna (a child at the time) and took him to Shōnai to become a servant.
Thereafter, he came into conflict with the Shimizu clan who were members of the Mogami family and, in 1565, at the Battle of Motoaikai, killed Shimizu Yoshitaka while temporarily advancing into the Murayama District of Dewa but withdrew in the face of a counterattack by the Mogami army.
On 5/19 of Eiroku 8 (1565), Ashikaga Yoshiteru, the thirteen shōgun of the Muromachi bakufu, was killed in an attack orchestrated by Matsunaga Hisamichi and Miyoshi Yoshitsugu. This is known as the Eiroku Incident. Based on an alliance with the Honjō clan, Yoshimasu then sought for independence from the Mogami clan. In 1568, he supported a rebellion by Honjō Shigenaga known as the Revolt of Honjō Shigenaga, but after Uesugi Kenshin ended attacks on the Honjō and turned his army toward the Daihōji, Yoshimasu surrendered. He then tendered his son, Daihōji Yoshiuji, as a hostage to Kenshin and settled. The following year, Yoshimasu was pressured to assign the headship of the clan to Yoshiuji and retired.
He died on 8/1 of Tenshō 9 (1581).