Saishin (also called Seijin) (March 12, 954 - July 14, 1030) was a monk of Shingon sect in the mid Heian period. His father was MINAMOTO no Masanobu, Sadaijin (Minister of the Left) (Some people say that his father was Imperial Prince Atsumi). He was also called Shingonin Sojo (high-ranking Buddhist priest), Kitain Daisojo (a Buddhist priest of the highest order), Sojo of Ninna-ji Temple, or Sojo of Kannon-in Temple.
He studied Shingon Esoteric Buddhism under Gakei and he also studied Hosso sect doctrine. He received the teachings of Buddhism from Kancho and was appointed to Gon-Risshi (generally in Shingon sect, fifteenth-ranking Buddhist priest, literally, "supernumerary master of discipline") in 989. He later successively held the positions of betto (the superior of a temple) of Todai-ji Temple, chori (the head priest) of Kaju-ji Temple, and To-ji Choja Homu (Director of Temple Affairs and the chief abbot of To-ji Temple). He rose to be a daisojo in 1019 and was given the Gissha no senji (special permission from the Emperor to enter the imperial palace in an ox-drawn cart) for the first time as a monk the next year, 1020. He often performed as a doshi (officiating monk) at Kanjo-daies and esoteric Buddhism services and served as a doshi at the funeral ceremony for FUJIWARA no Michinaga in 1027.