Seizon 成尊 (1012–1074) was a mid-Heian Japanese Shingon 真言 scholar-monk and lineage-master, founder of the Ono-ryū 小野流 sub-lineage of post-Kūkai Shingon transmission. He served as zasu of the Daigo-ji 醍醐寺 institutional complex and held the senior court-Buddhist rank of Sōjō 僧正 (“Bishop”), making him one of the most institutionally-influential Shingon figures of the mid-Heian period.
Seizon was the principal disciple of Ningai 仁海 (951–1046), founder of the Ono-ryū at Daigo-ji’s Mandara-ji 曼荼羅寺 sub-temple, and he consolidated his teacher’s lineage into the dominant medieval Shingon sub-lineage. The Ono-ryū rapidly produced major systematizers in subsequent generations — Saisen (濟暹, 1025–1115), Kakuban (覺鑁, 1095–1144), and many others — establishing the institutional and doctrinal framework of medieval Daigo-ji Shingon.
His KR6t0139 Zhēnyán fùfǎ zuǎnyào chāo 眞言付法纂要抄 (“Essential Compendium of the Mantra Dharma-Transmission”) is the foundational lineage-narrative text of the medieval Shingon school: a one-fascicle compressed account of the eight Esoteric patriarchs of the Shingon Dharma-transmission line (Mahāvairocana → Vajrasattva → Nāgārjuna → Nāgabodhi → Vajrabodhi → Amoghavajra → Huìguǒ → Kūkai), composed in response to an imperial request.
Seizon also was the principal kanjō-master responsible for the imperial Esoteric consecration of his generation, conferring denpō kanjō upon imperial princes and senior court figures and consolidating the Shingon school’s institutional relationship to the late-Heian imperial court.