Usui Sadamitsu (碓井貞光) was a warrior of the mid-Heian period. His official name was Taira no Tadamichi (平忠通).
According to Otogizōshi stories compiled several centuries later, Sadamitsu was a retainer of the Japanese legendary hero Minamoto no Raikō. Sadamitsu is known as one of The Four Guardian Kings under Raikō. Alternatively, in Konjaku Monogatari he is listed as one of three retainers to Raikō. Following the Tale of the Ground Spider within the tale of Raikō, Sadamitsu personally protected Raikō when he was suffering a mysterious ailment. Sadamitsu is also depicted in artwork and kabuki plays. At times Sadamitsu is depicted as female.
- Sato, Hiroaki (1995). Legends of the Samurai. Overlook Duckworth. pp. 61–64. ISBN 9781590207307.