Ennin (圓仁 or 円仁, 793 CE  or 794 CE – 864 CE), better known in Japan by his posthumous name, Jikaku Daishi (慈覺大師), was a priest of the Tendai school of Buddhism in Japan, and its third Zasu (座主, "Head of the Tendai Order"). Ennin was instrumental in expanding the Tendai Order's influence, and bringing back crucial training and resources from China particularly esoteric Buddhist training, and Pure Land teachings.
In 838, Ennin was in the party which accompanied Fujiwara no Tsunetsugu's diplomatic mission to the Tang dynasty Imperial court. The trip to China marked the beginning of a set of tribulations and adventures.
Initially, he studied under two masters and then spent some time at Wutaishan (五臺山; Japanese: Godaisan), a mountain range famous for its numerous Buddhist temples in Shanxi Province in China. Later he went to Chang'an (Japanese: Chōan), then the capital of China, where he was ordained into both mandala rituals. He also wrote of his travels by ship while sailing along the Grand Canal of China.
Ennin was in China when the anti-Buddhist Emperor Wuzong of Tang took the throne in 840, and he lived through the Great Anti-Buddhist Persecution of 842–846. As a result of the persecution, he was deported from China, returning to Japan in 847.
In 847 he returned to Japan and in 854, he became the third abbot of the Tendai sect at Enryakuji, where he built buildings to store the sutras and religious instruments he brought back from China. His dedication to expanding the monastic complex and its courses of study assured the Tendai school a unique prominence in Japan. While his chief contribution was to strengthen the Tendai tantric Buddhist tradition, the Pure Land recitation practices (nenbutsu) that he introduced also helped to lay a foundation for the independent Pure Land movements of the subsequent Kamakura period (1185–1333). Ennin also founded the temple of Ryushakuji at Yamadera.
He wrote more than one hundred books. His diary of travels in China, Nittō Guhō Junrei Kōki (入唐求法巡礼行記), was translated into English by Professor Edwin O. Reischauer under the title . Sometimes ranked among the best travelogues in world literature, it is a key source of information on life in Tang China and Silla Korea and offers a rare glimpse of the Silla personality Jang Bogo.