Lifespan: Meiō 1 (1492) (?) to 2/2 of Eishō 17 (1520)
Other Names: Dewa-tarō (common)
Rank: sengoku daimyō
Title: Master of the Office of Palace Repairs
Bakufu: Muromachi – Commissioner of Dewa
Father: Mogami Yoshiatsu
Siblings: Yoshisada, Nakano Yoshitatsu, Yachi Teruyuki
Wife: [Formal] Daughter of Yamanobe Naohiro, [Second] Daughter of Date Hisamune
Adopted Children: Yoshimori
Mogami Yoshisada served as a sengoku daimyō during the early Sengoku period. He was the ninth head of the Mogami clan of Dewa Province. He further served as the ushū-tandai, or Commissioner of Dewa, for the Muromachi bakufu. Meanwhile, his contemporary, Date Harumune, served as the oushū-tandai, or Commissioner of Mutsu, for the bakufu. Together, these two sengoku daimyō governed northern Japan.
Around 1492, Yoshisada was born as the eldest son of Mogami Yoshiatsu, a sengoku daimyō and the eighth head of the Mogami clan.
In 1504, following the demise of his father, Yoshiatsu, Yoshisada became the lord of Yamagata Castle. Later that year, Yoshisada learned of a succession dispute besetting his archrival – the Sagae clan. After launching a series of attacks, the two sides entered into a settlement and the Sagae came under the command of the Mogami. As a result of the fighting, an important mountain temple in Sagae known as the Jion Temple and lodges for the monks. Valuables were also lost, although the artifacts in the main hall were spared.
The daughter of Sagae Tomohiro, the thirteenth head of the clan, wed Yoshisada’s younger brother, Nakano Yoshitatsu. In 1512, the Mutō and Sagoshi clans clashed in the Shōnai area. Anticipating an advance by the prevailing party, Yoshisada established a base in Sagae. He also wed the daughter of Yamanobe Naohiro, a family member.
In 1508, the Date clan proceeded to Kaminoyama and, in 1514, invaded Yamagata. Yoshisada led an alliance comprised of influential branches of the Mogami including the Tendō, the Shimizu, and the Nobesawa along with the Sagae and others under their command to fight against Date Tanemune at Hasedō, but was defeated. Yamanobe Naohiro and Yoshikawa Masachika (members of the Sagae clan) were killed in action. Tanemune stationed Koyanagawa Chikatomo at Hasedō Castle, posing a major threat to Yamagata Castle. Consequently, Yoshisada was compelled to move to Nakano Castle. The next year, Yoshisada wed a younger sister of Tanemune and the two sides settled. Thereafter, however, owing to the influence of the Date clan, the power of the main branch of the Mogami clan receded. This became a key factor leading to the formation of the Eight Shields of the Mogami, an alliance of powerful kokujin, or provincial landowners, who were illegitimate branches of the Mogami led by the Tendō clan in Dewa.
In the midst of these circumstances, on 2/2 of Eishō 17 (1520), Yoshisada died. He did not have any children with wives from the Date clan so Tanemune intervened in the succession dispute. Two years later, Mogami Yoshimori, the grandson of Nakano Yoshitatsu (Yoshisada’s younger brother who inherited the Nakano clan), became Yoshisada’s successor as head of the Mogami clan at the age of two. Yoshisada did have a son with a wife from the Yamanobe clan, but there is a theory that he was sidelined by the Date family seeking to have the Mogami become a puppet regime.