Ensai (year of birth unknown – 877) was a priest of the Tendaishu sect of the early Heian period. He was from Yamato Province.
From his childhood, he studied under Saicho, the founder of the Nihon (Japanese) Tendaishu sect, and in 838 went to Tang (China) bearing with him 50 questions on the doctrine of the Tendaishu sect raised by Tendai zasu (head priest of the Tendai sect), Encho. He obtained Toketsu (a collection of questions on Tendai doctrine posed by Japanese Tendai monks with answers by Chinese Tiantai masters) from Koshu and Iken of Mt. Tendai (Mt. Tiantai) and sent it to Japan through his disciple, Ninko. Remaining in Tang even after that, with his scholarship he accepted becoming a believer in Buddhism by Xuanzong (Tang Dynasty), and in 855 he received kanjo (a ceremony to be the successor) along with Enchin from Hazzen of Qinglongsi Temple of Changan. Meanwhile he had received two remittances of gold from the Japanese Imperial Court. In 877 on his way back to Japan, he died an unnatural death from a shipwreck. Furthermore, there have been rumors that while he stayed in Tang he had some destructive bad behaviors.
Besides, while he stayed in Tang he encountered persecution of Buddhists in the Huichang era, and as well as other priests he was forced to return to secular life and even had a wife and children.
This can be known to Japan as his 'destructive bad behavior.'
Ennin who stayed in Tang during the same period was also forced to return to secular life (it was just before his return to Japan that Ennin took the tonsure again).
(As for Ennin, unlike Ensai, returned to Japan safely.)