Born in 1200, believed to be the son of court minister Kuga Michichika (disputed). Orphaned young, he entered Mt. Hiei at 13 and studied Tendai, but could not resolve his question: if all beings inherently possess the Buddha-nature, why must Buddhas still undergo training? He descended to Kenninji and studied under Myozen, a disciple of Eisai. In 1223 he crossed to Song China with Myozen and trained in shikantaza (just sitting) under master Rujing at Tiantong Monastery, attaining the enlightenment he called "dropping off body and mind" and receiving Rujing's certification. Returning to Japan in 1227, he transmitted Soto Zen with shikantaza as its essence. He founded Koshoji in Uji (1233) and, after briefly visiting Kamakura at the shogunate's invitation in 1247—then declining to remain, unwilling to associate with political power—he moved to Echizen (present-day Fukui) and built Eihei-ji in 1244. His major work "Shobogenzo" (75 volumes) is regarded as the most profound philosophical text in Japanese history. He died in 1253 at age 54.

An uncompromising seeker of truth. Indifferent to power and fame, he pursued zazen and practice single-mindedly. A precise thinker of unmatched philosophical depth in Japanese Buddhism.

Soto Zen became Japan's largest Zen school; Eihei-ji remains an active training monastery. "Shobogenzo" is regarded as Japan's greatest philosophical work, influencing modern philosophers like Nishida Kitaro.