Eiku (year of his birth is unclear - 1179) was a Buddhist monk of the Tendai Sect in the late Heian period. Although his father is said to have been Dajo daijin (Grand Minister) FUJIWARA no Koremichi, it is not certain. His bogo (another name of a priest) was Jiganbo.
After receiving Mahayana precepts from Ryonin, he studied Mikkyo (Esoteric Buddhism) as well as Jodo Sect and resided in Saito Kurodani in Mt. Hiei. He was well known for his lecture of 'Ojoyoshu' (Buddhism book), and was praised as the most prominent learned priest of Mahayana precepts and Mikkyo in Mt. Hiei and was regarded as the founder of Kurodani school of Mahayana precepts. He served in a role as mentor within jukai (handing down the precepts) for many court nobles and was noted for being in a mentor-family relation with the Kuga family. When Honen, the founder of Nihon Jodo Sect, became his disciple in 1150, he taught Honen Tendai doctrine as well as Jodo doctrine and granted Mahayana precepts.