Kōshū 光宗 (1276–1350) was a late-Kamakura / Nanboku-chō Tendai master, resident of the Saitō 西塔 precinct of Mt. Hiei, and one of the most prolific Buddhist authors of medieval Japan. He is principally famous as the compiler of the monumental Xīlán shíyè jí 溪嵐拾葉集 (Keiran-shūyōshū, “Gathered Leaves from the Mountain Stream-Mist” — KR6t0110, 116 fascicles), the most extensive encyclopedic-doctrinal compendium of medieval Japanese Tendai.
Kōshū was a student of Sonpen 尊辨 of the Kurodani 黑谷 / Saitō lineage, and his work spans virtually every doctrinal, ritual, biographical, hagiographical, and folkloric concern of medieval Hiei-zan. The Keiran-shūyōshū records: Taimitsu procedural variants and kuden; doctrinal-philosophical debates (hongaku 本覺 thought, sokushinjōbutsu); the cult of Sannō 山王 (the Hiei tutelary deities); Tendai precept-discipline; biographical anecdotes of patriarchs; medical, calendrical, and divinatory lore; and an enormous corpus of medieval Hiei-zan folkloric and kuden material that is unrecoverable from any other source.
Modern scholarship treats Kōshū as the principal documentary witness for medieval Tendai kenmitsu thought, and the Keiran-shūyōshū is the single most-cited primary source in the modern study of medieval Japanese Buddhist intellectual and ritual history.
He should be distinguished from the late-Tang Buddhist 光宗 / 光定 (active 9th c.) and from the Tendai Shingi 真盛 (眞盛) line Kōshū of a later generation.