Saichō (最澄; Japanese pronunciation: [saꜜi.tɕoː], September 15, 767 – June 26, 822) was a Japanese Buddhist monk credited with founding the Japanese Tendai school of Buddhism. He was awarded the posthumous title of Dengyō Daishi (伝教大師).

Recognized for his significant contributions to the development of Japanese Buddhism, Saichō is most famous for introducing the Chinese Tiantai school to Japan, which he adapted into the Tendai tradition. Saichō traveled to Tang China in 804, where he studied the Chinese Tiantai school (along with other traditions). After returning to Japan, he founded the temple and headquarters of Tendai at Enryaku-ji on Mount Hiei (near the capital of Kyoto), which became the center of Tendai practice and a major institution in the history of Japanese Buddhism.

Saichō emphasized the integration of the Tiantai teachings on meditation, study, precepts, and ritual practice, with the mantrayana practices of Chinese Esoteric Buddhism. He also worked to establish a new ordination system which was based on the bodhisattva precepts, rather than the traditional monastic rule (Vinaya) precepts.

Saichō's Tendai school laid the groundwork for the development of later Japanese Buddhist traditions, including Pure Land, and Zen Buddhism.

Saichō was born in the year 767 in the city of Ōmi, in present Shiga Prefecture, with the given name of Hirono. According to family tradition, Saichō's ancestors were descendants of emperors of Eastern Han China; however, no positive evidence exists for this claim. The region where Saichō was born did have a large Chinese immigrant population, so Saichō likely did have Chinese ancestry.

During Saichō's time, the Buddhist temples in Japan were officially organized into a national network known as the provincial temple system, and at the age of 13, Saichō became a disciple of one Gyōhyō (722–797, 行表). He took tonsure as a novice monk at the age of 14 and was given the ordination name "Saichō". Gyōhyō in turn was a disciple of Dao-xuan (702–760, 道璿, Dōsen in Japanese), a prominent monk from China of the Tiantai school who had brought the East Mountain Teaching of Chan Buddhism, Huayan teachings and the Bodhisattva Precepts of the Brahmajala Sutra to Japan in 736 and served as the "precept master" for ordination prior to the arrival of Jianzhen.

By the age of 20, he undertook the full monastic precepts at the Tōdai-ji, thus becoming a fully ordained monk in the official temple system. A few months later he abruptly retreated to Mount Hiei for an intensive study and practice of Buddhism, though the exact reason for his departure remains unknown. Shortly after his retreat, he composed his Ganmon (願文; "Saichō's Prayer") which included his personal vows to:

In time, Saichō attracted other monks both on Mount Hiei, and from the Buddhist community in Nara, and a monastic community developed on Mount Hiei, which eventually became Enryaku-ji. Saichō was said to have carved an image of the Bhaiṣajyaguru and enshrined it. Additionally, he lit a lamp of oil before the Buddha and prayed that the lamp would never be extinguished. This lamp is now known as the Fumetsu no Hōtō (不滅の法灯; "Inextinguishable Dharma Lamp") and has remained lit for 1200 years.