Zhiyi (Chinese: 智顗; pinyin: Zhìyǐ; Wade–Giles: Chih-i; Japanese pronunciation: Chigi; Korean: 지의; 538–597 CE) also called Tiantai Dashi (天台大師) and Zhizhe (智者, "Wise One"), was a Chinese Buddhist monk, philosopher, meditation teacher, and exegete. He is considered to be the founder of the Tiantai Buddhist tradition, as well as its fourth patriarch. Śramaṇa Zhiyi is widely regarded as one of the most influential figures in the development of East Asian Buddhist thought and practice. As the first major Chinese Buddhist thinker to construct a comprehensive religious system based primarily on Chinese interpretations, Zhiyi played a crucial role in synthesizing various strands of Mahayana Buddhism into a unique coherent framework. According to David W. Chappell, Zhiyi "has been ranked with Thomas Aquinas and al-Ghazali as one of the great systematizers of religious thought and practice in world history."

Zhiyi relied on the teachings of the Lotus Sutra as the main basis for his system, though he also drew on numerous texts, such as the works of Nagarjuna. One of his central innovations was the Threefold Truth, which unifies the truths of emptiness, and provisional existence, with a holistic third truth: the middle. Zhiyi also developed an influential interpretation of the Lotus Sutra, which he used to interpret all other Mahayana Buddhist teachings. Zhiyi's comprehensive work on Buddhist practice, the Mohe Zhiguan (Great Cessation-Contemplation), outlines step-by-step instructions for Buddhist meditation and cultivation, combining traditional Indian methods with unique innovations. This text continues to serve as an influential guide for meditators across East Asian Buddhist traditions.

Zhiyi's Tiantai school became one of the most significant Buddhist traditions in imperial China, and its teachings later spread to Korea, Japan and Vietnam. Zhiyi's synthesis of doctrine and practice remains a cornerstone of East Asian Buddhist philosophy. His three great works, the Great Cessation-Contemplation, the Profound Meaning of The Lotus Sutra, and the Words and Phrases of The Lotus Sutra are the foundational treatises for the Tiantai, Tendai (Japanese) and Cheontae (Korean) traditions. Zhiyi's works also influenced other Buddhist traditions, such as Chan/Zen, Pure Land and Nichiren Buddhism, and continue to be studied by Asian Buddhists for their depth, clarity, and systematic approach to Buddhist thought. His system provides a universalist Mahayana framework which allowed it to easily adapt to new times and cultures.

Born with the surname Chen (陳) in Huarong District, Jing Prefecture (now Hubei), Zhiyi lost his parents and hometown of Jiangling to the Western Wei army when he was just seventeen. He subsequently became a Buddhist monk at eighteen. As a young monk, he studied Mahayana sutras and Vinaya, and also recited the Threefold Lotus Sutra, under the guidance of Vinaya master Huikuang (d.u.).

At 23, Zhiyi met his most important teacher, Nanyue Huisi (515–577 CE), a meditation and Lotus Sutra master who would later be listed as Zhiyi's predecessor in the Tiantai lineage. On first meeting, Huisi is said to have greeted Zhiyi as an old friend, since he recognized that they had both been present in the Lotus Sutra assembly at Vulture's Peak. Under Huisi's tutelage (from 560 to 567), Zhiyi practiced the Lotus Samadhi Repentance, the Pratyutpanna Samadhi, the suiziyi sanmei (the samādhi of freely flowing thoughts), the Lotus Sutra based "four practices of ease and bliss" (si anle xing), and the meditative recitation of the Lotus Sutra.

In 567, Zhiyi (now 30 and approved by Huisi as his successor) traveled with several followers to Waguansi monastery at the Southern capital of Jinling (Jiangsu) to give teachings on the Lotus Sutra and the Dazhidu lun. He spent eight years at the capital teaching. After eight years, feeling that his efforts in the capital were not effective in bringing people to the true Dharma, he moved to Tiantai mountain (Zhejiang province) in 575, where he would remain for eleven years studying and practicing. This move came around the same time as the second great persecution of Buddhism by Emperor Wu (r. 561–578) was raging in the north. Zhiyi also built a monastery on mountain Tiantai, which was later named Xiuchansi (修禪寺).

In 585 Zhiyi returned to the capital of Jinling as requested by the king of Chen. It is here that he would give a series of lectures on the Lotus Sutra which would later be edited by his disciple into an influential commentary, the Miao Falianhua wenju (妙法蓮華文句; "Words and Phrases of the Wondrous Dharma Lotus Flower", or Fahua wenju for short, T. 1718). Zhiyi also acted as preceptor of the bodhisattva precepts to the future Emperor Yang (r. 604-617) of Sui, at the prince's request, who then granted Zhiyi the title of Dashi "Zhizhe" (Great Master Wise Man). He then founded another monastery in his native Jingzhou which later came to be called Yuquansi (玉泉寺).

In the latter part of his life, he gave other lectures which would become his other great works, the Fahua xuanyi (Profound Meaning of The Lotus Sutra) and the Mohe zhiguan (Great Calming and Insight). He also wrote two commentaries on the Vimalakīrtinirdeśa just before the end of his life in 597.