Keiso (955 - January 25, 1020) was a priest of the Tendai sect during the mid Heian period. His secular surname was Nakahara.
Keiso studied the Tendai doctrine under Yokei of Onjo-ji Temple. When the disciples of Enchin were kicked out of Mt. Hiei in 993, Keiso moved first to Iwakura Daiun-ji Temple and then to Onjo-ji Temple. Many scholar priests were attracted to Keiso for his deep knowledge and virtue and gathered around him, building a basis for prosperity of the Tendai sect at Onjo-ji Temple. In 997, upon the order of the Imperial Court, Keiso criticized 'Ryujo jobutsugi' (Principle of Attaining Buddhahood for Ryujo) sent from Sung Dynasty. In 1017 Keiso performed Hokke Juko (lectures on Hoke-kyo Sutra in eight volumes, Kai-kyo [sutra read before the main sutra] and Kechi-kyo [sutra read after the main sutra]) at a memorial service for Enchin. Keiso was known for his deep knowledge and was respected by Zoga and Genshin (priest).