Gasan Jōseki 峨山韶碩 (Kenji 2 → 1276; Jōji 5 / Shōhei 21 / 1366-11-08), Late-Kamakura → Nanbokuchō Japanese Sōtō-Zen master, principal dharma-heir of 瑩山 Keizan Jōkin (1268–1325) and second-generation abbot of Sōji-ji 總持寺 in Noto. Style-name (字) Gasan 峨山; dharma-name Jōseki 韶碩. Posthumous title Myōkaku Tennō Kokushi 妙覺天皇國師, with later additional titles. Native of Noto 能登 province (modern Ishikawa).
Tonsured young at Yōkō-ji 永光寺 under Keizan; received Keizan’s transmission as the principal heir of the Sōji-ji-line. Succeeded as second abbot of Sōji-ji on Keizan’s death (1325) and led the temple for the next four decades — the formative era in which Sōji-ji became the dominant Sōtō-Zen institution outside Eihei-ji. Trained an extraordinary cohort of senior disciples — the Five Disciples (峨山五哲 Gasan no goteki) — through whom every later Sōtō sub-lineage descends:
- Tsūgen Jakurei 寂靈 (1322–1391) — Tsūgen-ha
- Mutan Sokan 無端祖環 (?–1397) — Mutan-ha
- Daitetsu Sōrei 大徹宗令 (1333–1408) — Daitetsu-ha
- Jippō Ryōshū 良秀 (1318–1405) — Jippō-ha
- Genō Hon’eki 源翁本益 (1329–1400) — Genō-ha
These five lineages became the institutional structure of medieval and early-Edo Japanese Sōtō, and through them Sōji-ji-line Sōtō spread throughout the Japanese countryside. Gasan is conventionally credited as the principal institution-building patriarch of the Sōji-ji branch — Keizan provided the doctrinal and liturgical framework, Gasan and his five disciples translated this into a working temple-network. His own recorded sayings, the Gasan Wajō goroku 峨山和尚語録, survive separately (not in the present batch).