Zhiyi (Chinese: 智顗; pinyin: Zhìyǐ; Wade–Giles: Chih-i; Japanese pronunciation: Chigi; Korean: 지의; 538–597 CE) also called Zhizhe (智者) or Chen De'an (陳德安), was the fourth patriarch of the Tiantai tradition of Buddhism in China.

Zhiyi is famous for being the first in the history of Chinese Buddhism to elaborate a complete, critical and systematic classification of the Buddhist teachings.

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.

At 23, he received his most important influences from his first teacher, Nanyue Huisi (515–577 CE), a meditation master who would later be listed as Zhiyi's predecessor in the Tiantai lineage.

Chappell holds that Zhiyi: "...provided a religious framework which seemed suited to adapt to other cultures, to evolve new practices, and to universalize Buddhism.

Zhiyi's Xiao Zhiguan states:[9]There are many ways to enter the true reality of nirvana, but none that is more essential or that goes beyond the twofold method of cessation-and-contemplation.

The reason is that "cessation" is the preliminary gate for overcoming the bonds [of passionate afflictions]; "contemplation" is the proper requisite for severing delusions.

[13] The Mohe Zhiguan is the magnum opus of Zhiyi's maturity and is held to be a "grand summary" of the Buddhist Tradition according to his experience and understanding at that time.

[14] The text of the Mohe Zhiguan was refined from lectures Zhiyi gave in 594 in the capital city of Jinling and was the sum of his experience at Mount Tiantai c.585 and inquiry thus far.

Zhiyi taught the principle of Three Thousand Realms in a Single Thought Moment (Chinese: 一念三千; Pinyin: Yīniàn Sānqiān) in his 'Great Concentration and Insight', based on the Lotus Sutra.