Chōen 長宴 (1016–1081) was a major mid- to late-Heian Tendai esoteric (Taimitsu 台密) master, foundational figure of the Renge-ryū 蓮華流 (Lotus-lineage) sub-line within the broader Kawa-ryū 川流 (River-lineage) tradition on Mt. Hiei. He is known by the sobriquet Kawa-no-ajari 川阿闍梨 (“the Ācārya of the River”).
Chōen was a disciple of Kōkei 皇慶 (皇慶) and absorbed the Tani-ryū procedural-doctrinal corpus before establishing his own scholastic line. His project was the comprehensive collation of the Tani-ryū oral-transmission heritage: he gathered, sifted, and committed to writing the kuden of his teacher Kōkei and of the broader mid-Heian Yokawa Taimitsu transmission, producing the forty-volume Sìshí tiè jué 四十帖決 — the most extensive and important kuden compendium of mid-Heian Tendai esotericism.
His KR6t0108 Sì-shí tiè jué (15 fascicles in the Taishō edition, originally 40 tiè “fascicle-folders”) is the canonical Tani-ryū / Kawa-ryū kuden compendium and remained the principal medieval Tendai-esoteric reference until the rise of the Edo-period systematizations. The work covers Garbhadhātu and Vajradhātu kanjō procedure, doctrinal questions on the Mahāvairocana-sūtra and Susiddhi-sūtra, ritual implementation of various deity-specific rites (Acalanātha, Mañjuśrī, etc.), and esoteric textual exegesis.
He should not be confused with the later Pure-Land Chōen nor with the Shingon-school monk of the same name.