Ninsen 忍仙 (dates unknown, fl. 1308–1328) was a Kamakura-period Buddhist Vinaya master, very likely affiliated with the Saidai-ji 西大寺 or Tōshōdai-ji 唐招提寺 lineage of the precept revival movement initiated by Kakujō 覺盛 and Eison 叡尊 (覺盛, 叡尊). The colophon of his single preserved work, KR6t0057 Lǜ-zōng xíng-shì mù-xīn chāo (1328), reports that the work was composed over a 21-year span from autumn Enkyō 1 (1308) to spring Karyaku 3 (1328), in order to record the standing-orders of his preceptor (hé-shàng zhī yán-pàn) — confirming that he was a junior monk recording the operational practice of a senior Vinaya teacher. He identifies himself simply as 釋沙門 (Śākya-śramaṇa) without lineage qualifier.
His work represents one of the most thorough Kamakura-period systematizations of the practical operational procedures (xíng-shì) of monastic Vinaya — including the proxy-consent system (yù), boundary-establishment (jié-jiè), and uposatha / pravāraṇā — and integrates the Brahmajāla bodhisattva-precept poṣadha tradition into the Sì-fēn-lǜ (Dharmaguptaka) operational framework characteristic of the Kamakura Vinaya synthesis.
No CBDB or DILA Authority record currently locates this Ninsen; he should not be confused with the slightly later Sōtō Zen monk Ninsen 忍先 of the Eihei-ji lineage.