Tenkai (1536 – 1643) was a Japanese Tendai Buddhist monk of the Azuchi-Momoyama and early Edo periods. He achieved the rank of Daisoujou, the highest rank of the priesthood. His Buddhist name was first Zuifuu (随風), which he changed to Tenkai in 1590. Also known as Nankoubou Tenkai (南光坊 天海), he died in 1643, and was granted the posthumous title of Jigen Daishi (慈眼大師) in 1648.
Tenkai was at Kita-in (北院) in Kawagoe in 1588, and became abbot in 1599. He was on the staff of Tokugawa Ieyasu, and served as a liaison between the Tokugawa Shogunate and the Imperial Court in Kyoto. One of his projects was the rebuilding of Enryaku-ji, which had been devastated by Oda Nobunaga. He also revitalized Kita-in, and changed the characters of its name to "喜多院". Nearing death in 1616, Tokugawa Ieyasu entrusted Tenkai with his last will regarding matters of his funeral and his posthumous name. Tenkai selected gongen rather than myoujin, and after death Ieyasu became known as Toushou Daigongen.
Tenkai established Kan'ei-ji in 1624.Tenkai continued to serve as a consultant to the next two Tokugawa shoguns. In 1624, retired shogun Tokugawa Hidetada and ruling shogun Tokugawa Iemitsu asked him to establish Kan'ei-ji, a Buddhist temple to the northeast of Edo Castle in Ueno.
There are several theories concerning his early life. Some fiction writers postulate that he was in actuality Akechi Mitsuhide. It is not certain whether Mitsuhide died at the Battle of Yamazaki or not, and some suppose that he survived and began a new life as the priest Tenkai.