Kaiyo (1036-September 6, 1112) was a Buddhist monk from the end of the Heian Period (the cloister government period). The fourth (or fifth) son of MINAMOTO no Yoriyoshi. He entered into priesthood at Hieizan Enryaku-ji Temple, and took Iyo Ajari as his religious name. MINAMOTO no Yoshiie, MINAMOTO no Yoshitsuna, and MINAMOTO no Yoshimitsu were his brothers.
He maintained an intimate relationship with his immediate elder brother Yoshimitsu, and when Yoshimitsu murdered his nephew, MINAMOTO no Yoshitada, in 1109, it is said that he was deeply involved in its planning and execution. TAIRA no Narimoto, although the architect of the assassination of Yoshitada, was instructed by Yoshimitsu to hide after the crime at Kaiyo's wing, and Kaiyo, after receiving him, buried him alive. However, after receiving Narimoto, Kaiyo buried him alive and killed him. It is said that the brothers Yoshimitsu and Kaiyo got together and conspired this act from the beginning in order to silence him and avoid detection of the case.