The Koide clan originated from the Koide township in the Ina District of Shinano Province. The Koide were a branch of the Nikaidō clan descended from the Fujiwara-Nanke, becoming a daimyō family in the Azuchi-Momoyama and Edo periods. Their descendants settled in Nakamura in Owari Province. While Yoshimasa was born in Owari, he was later granted fiefs in Tajima and Izumi provinces.
Lifespan: Eiroku 8 (1565) to 2/29 of Keichō 18 (1613)
Rank: bushō, daimyō
Title: Junior Fifth Rank (Lower), Governor of Shinano, Governor of Harima, Governor of Yamato
Lord: Toyotomi Hideyoshi → Toyotomi Hideyori
Domain: Lord of Tajima-Izushi → lord of Izumi-Kishiwada
Father: Koide Hidemasa
Mother: Eishōin (younger sister of mother of Toyotomi Hideyoshi)
Siblings: Yoshimasa, Hideie, Nichijū, Mitsutada, Shigekata, Hidekiyo
Wife: [Formal] Chōshun-in (daughter of Itō Haruaki)
Children: Daughter (second wife of Katō Sadayasu), Yoshihide, Yoshichika, Yoshikage, Yoshinari, daughter (second wife of Matsudaira Tadaakira)
Adopted Children: Kinoshita Toshisada
Koide Yoshimasa served as a bushō and daimyō during the Azuchi-Momoyama and early Edo periods. He served as the lord of the Izushi domain in Tajima Province and, later, as the second lord of the Kishiwada domain in Izumi Province. He was the second head of the Koide family in the Izushi domain. He had the common name of Kosaiji.
In 1565, Yoshimasa was born as the eldest son of Koide Hidemasa in Nakamura in the Aichi District of Owari Province. His mother was Eishōin. She was the younger sister of Ōmandokoro, the mother of Toyotomi Hideyoshi so, from early on, Yoshimasa served as an umamawari, or member of the cavalry, for Hideyoshi. Hideyoshi was an older cousin.
In 1590, he served in the Conquest of Odawara. In 1591, he was granted a fief in Izumi Province. In 1593, he was conferred the title of Junior Fifth Rank (Lower) and Governor of Shinano and granted a fief of 20,000 koku at Arikoyama Castle in Tajima Province. In 1594, Yoshimasa was responsible for the construction of Fushimi Castle. In the sixth month of the same year, his fief was increased by 20,000 koku, yielding a total fief of 40,000 koku. In 1595, he was transferred to Arikoyama Castle with a fief of 60,000 koku. Yoshimasa, together with his father, Hidemasa, governed a fief of 90,000 koku.
In the eighth month of 1598, following the death of Hideyoshi, he received a Kunikage sword as a keepsake. According to a license with a seal written by Hideyoshi shortly before his death, Yoshimasa was assigned an important role to guard the main rear gate to the inner citadel and the Aoyaguchi gate to Ōsaka Castle.
In the first month of 1600, in an ordinance jointly signed by three magistrates (Natsuka Masaie, Mashita Nagamori, and Maeda Gen’i), Yoshimasa and Katagiri Katsumoto are identified as those responsible to manage the cleaning and repair of Ōsaka Castle. Meanwhile, in earlier written rules, Yoshimasa was not included among the group of guards under Toyotomi Hideyori.
During the Battle of Sekigahara in the ninth month of 1600, Yoshimasa and his father, Hidemasa, joined the Western Army to provide security for Noguchi and Shinbori on the Ōsaka Plains. Yoshimasa participated in an attack against Hosokawa Yūsai at Tanabe Castle in Tango Province, an event known as the Siege of Tanabe Castle. His younger brother, Hideie, affiliated with the Eastern Army and served in the main Battle of Sekigahara, so, after the war, he received recognition of his rights to his fief of 60,000 koku.
In 1603, Hideie preceded his father in death from illness. After Hidemasa died the following year, Yoshimasa moved to his father’s former territory at Kishiwada Castle with a fief of 30,000 koku. His eldest, son, Koide Yoshihide, took over the territory in Izushi.
In 1613, Yoshimasa died in the year before the Siege of Ōsaka at the age of forty-nine. He was buried at the Kzō Temple on Mount Kōya in Kii Province.