Ishida Mitsunari (石田 三成, 1559 – November 6, 1600) was a Japanese samurai and military commander of the late Sengoku period of Japan. He is probably best remembered as the commander of the Western army in the Battle of Sekigahara following the Azuchi–Momoyama period of the 16th century. Also known by his court title, Jibu-no-shō (治部少輔).
He was born in the north of Ōmi Province (which is now Nagahama city, Shiga Prefecture), and was the second son of Ishida Masatsugu, who was a retainer for the Azai clan. His childhood name was Sakichi (佐吉). The Ishida withdrew from service after the Azai's defeat in 1573. According to legend, he was a monk in a Buddhist temple before he served Toyotomi Hideyoshi, but the accuracy of this legend is doubted since it only came about during the Edo period.
Mitsunari met Toyotomi Hideyoshi when the former was still young and the latter was the daimyō of Nagahama. When Hideyoshi engaged in a campaign in the Chūgoku region, Mitsunari assisted his lord in attacks against castles like the Tottori Castle and Takamatsu Castle (in present-day Okayama).
After Hideyoshi seized power, Mitsunari became known as a talented financial manager due to his knowledge and skill at calculation. From 1585 onward, he was the administrator of Sakai, a role he took together with his elder brother Ishida Masazumi. He was appointed one of the five bugyō, or top administrators of Hideyoshi's government. Hideyoshi made him a daimyō of Sawayama in Ōmi Province, a five hundred thousand koku fief (now a part of Hikone). Sawayama Castle was known as one of the best-fortified castles during that time.