Buddhist monk, founder of the eclectic Tiantai (Japanese: Tendai) Buddhist sect, which was named for Zhiyi's monastery on Mount Tiantai in Zhejiang, China. His name is frequently but erroneously given as Zhikai.

Orphaned at age 17, Zhiyi turned to monastic life and was a disciple of the great Buddhist master Huisi from 560 to 567. From his first visit to Nanjing (567) until his death, Zhiyi was intimately associated with the imperial government, first with the Chen dynasty in southern China—one of the Southern Dynasties—and then with the Sui dynasty, which eventually reunified the country.

Confronted with the many divergent varieties of Buddhist thought that existed in his time, Zhiyi exhibited skill at compromise and classification. He regarded all the varieties of Buddhist doctrine as true and assumed they had all been present in the mind of Shakyamuni (the historical Buddha) from the time of his enlightenment. According to Zhiyi, the Buddha unfolded his teachings gradually in five periods, taking into account the capacity of his listeners: as they became more enlightened, they could absorb progressively more profound doctrines. In the fifth and final period the Buddha preached the Saddharmapuṇḍarīka-sūtra (Lotus Sutra) ( Lotus Sutra), which Zhiyi helped establish as the most popular scripture of east Asia.

He criticized both those who indulged in a purely intellectualized Buddhism and those who in reaction practiced a religion without a theological base. For him, study and contemplation were both indispensable for religious enlightenment. His sect, which claimed more than 5 million adherents in Japan in the early 21st century, was the leading sect in China in the 8th and 9th centuries.