Yoshimasa was the son of Kiso Yoshiyasu and a vassal of Takeda Shingen. He held Fukushima in the Kiso region of Shinano. He was married to one of Shingen's daughters but deserted the Takeda cause in 1582, severing his ties with Katsuyori in favor of Oda Nobunaga. He held off a Takeda army sent to bring him to submission and provided the Oda with assistance in their invasion of Kai and Shinano soon afterwards. He was later deprived of his holdings by Toyotomi Hideyoshi.
- Initial text from Sengoku Biographical Dictionary (Samurai-Archives.com) FWSeal & CEWest, 2005