The Mogami clan (最上氏) was an important Japanese clan, descended from a branch of the Ashikaga clan. During the Sengoku period they ruled the province of Dewa, now located in Akita Prefecture.
The Mogami were descended from the Shiba clan, a branch of the Ashikaga family. In 1354, Shiba Iekane (斯波家兼) on the orders of Ashikaga Takauji, fought against an army of the Southern Court (南朝) in the Ōu (奥羽) region, now Tōhoku.
In 1356, Iekane sent his son Shiba Kaneyori (斯波兼頼) to the Yamagata Basin to face the army of the Southern Court. Kaneyori built Yamagata Castle in 1360 and won against the Southern Court Army in 1367.
After the victory, he settled in the castle and took the name "Mogami" from the city in Dewa province.
In the early years the Mogami clan expanded its territories by giving the sons of the masters many lands. Their sons in turn became important vassals of the Mogami clan and dominated their lands using these blood family connections.
During the Mogami Mitsuie (最上満家) years, however, the clan lost power due to internal conflicts and deteriorating family relationships.
Eventually Mogami Yoshisada (最上義定) was defeated by Date Tanemune in 1514, and after many battles and bloodshed the Mogami clan was subdued by the Date clan. After the death of Yoshisada, Mogami Yoshimori (最 義守), who was two years old at the time, became head of the clan in 1522.
In 1542, a war broke out between Date Tanemune and his son Harumune; the Date clan was weakened by what is called the Tenbun (天文の乱) conflict, and Yoshimori took advantage of the situation by making the Mogami clan independent of the Dates once again.
Under the leadership of Mogami Yoshiaki the domain grew enormously expanded Mogami territories. He was one of the best generals of the Sengoku period, defeating his neighboring enemies one after another. As a result he was rewarded with 200,000 koku by Toyotomi Hideyoshi after the siege of Odawara in 1590.
Yoshiaki supported Tokugawa Ieyasu at the outbreak of the Sekigahara campaign in 1600, defending his territory from the attack of a huge army of the Uesugi clan in the Shōnai area.
As a reward, he was given 570,000 koku by Tokugawa Ieyasu at the end of the Battle of Sekigahara, and the Mogami fief became the fifth largest in Japan, excluding the Tokugawa fiefdom.
Later Yoshiaki was able to divert and control the waters of the Mogami River both to allow navigation and to improve the irrigation of rice fields. He also rebuilt and expanded Yamagata Castle and the surrounding city.
Yoshiaki died in 1614 at Yamagata Castle. In 1622 the Mogami territories were confiscated by the Tokugawa shogunate because there were many internal struggles for control of the clan.
They subsequently became kōke and continued to exist.