Saiko (year of birth unknown - July 7, 1177) was a monk and a the Retired emperor Goshirakawa's trusted vassal who lived in the late Heian era. His secular name was FUJIWARA no Moromitsu. He was a son of Tamemitsu OE, who was a member of the ruling family in Awa Province, and was later adopted by FUJIWARA no Ienari.
Some say that he was a child of Shinzei's menoto (a woman providing breast-feeding to a highborn baby). He was a son of Tamemitsu OE of the Oe Daiguji (Shinto priest of Inbe-jinja Shrine in Oe County) family and was originally a Zaichokanjin (local official in the Heian and Kamakura periods) of Awa Province; however, he then was adopted by FUJIWARA no Ienari, became a retainer of Shinzei who was supposedly his foster brother, and was promoted up to Saemon no jo (third-ranked officer of the Left Division of Outer Palace Guards). After the death of Shinzei in the Heiji Disturbance, he entered the priesthood and started to call himself Saiko. He later served the Emperor Goshirakawa and was called the most trusted vassal. Saiko joined a conspiracy by FUJIWARA no Narichika, Shunkan, and Yukitsuna TADA to beat the Taira clan, and became the mastermind of the closed discussion at the lodge in Shishigatani (the Shishigatani plot).
His children, FUJIWARA no Morotaka and FUJIWARA no Morotsune, caused a conflict with the members of Enryaku-ji Temple and this developed into a large social event in which monks of Hiei-zan Mountain made a petition. Morotaka and Morotsune were punished, and Saiko then falsely charged Myoun of Tendai-zasu (head priest of the Tendai sect) so that the Cloistered Emperor Goshirakawa removed him from the Tendai-zasu (head priest of the Tendai sect) position, tortured him, and deported him to Izu Province. Myoun however was rescued by monk-soldiers in the middle of his deportation, and Saiko then recommended the Cloistered Emperor Goshirakawa punish Myoun harshly.
In 1177, Yukitsuna TADA told TAIRA no Kiyomori about the Shishigatani plot during the conflict with the Hiei-zan Mountain monks. Saiko sought refuge with the Cloistered Emperor Goshirakawa but was captured. Kiyomori stomped on Saiko's face and condemned him, but Saiko cursed the Taira clan. Kiyomori got angry and tortured Saiko, and finally had him beheaded at Gojo Nishi Suzaku.