- Born: 1489
- Died: 1564
- Sons: Usami Katsuyuki
- Titles: Suruga no kami
- Other names: Sadamitsu (宇佐美定満), Sadakatsu
- Distinction: Uesugi retainer
The son of Usami Takatada, he was long a notable retainer of the Nagao clan of Echigo. He helped convince the young Uesugi Kenshin (then known as Nagao Kagetora) to rebel against his elder brother Harukage and led troops for him during the resulting civil war. In 1564 he is reputed to have murdered Nagao Masakage on Kenshin's orders, though given that he himself appears to have died (of drowning) during the same incident makes this somewhat odd. According to one story, Masakage's son, Kagekatsu, held a resentment against the Usami for this, and when he assumed command of the Uesugi in 1579, compelled Sadayuki's son Katsuyuki to flee Echigo.
- Initial text from Sengoku Biographical Dictionary (Samurai-Archives.com) FWSeal & CEWest, 2005