Shinsei (March 8, 1443 - - April 4, 1495) was a priest of the Tendai sect during the Sengoku period (period of warring states). His shigo (posthumous title) was Enkai Kokushi (the Most Reverend Priest, Enkai) and Jisetsu Daishi (the Great Teacher, Jisetsu). He came from Ise Province and his father was KOIZUMI Sakon no jo Fujiyoshi (Lieutenant of the Left Division of Inner Palace Guards, Fujiyoshi KOIZUMI). Shinsei is the founder of the Shinsei School of the Tendai Sect (present-day Tendai Shinsei sect).
He studied learning of religious doctrines under Keishu at the Saito (Western section) of Mt. Hiei. Later he retired in Kurotani Seiryu-ji Temple at the Saito of Mt. Hiei where he chanted the Invocation of Buddha's Name based on 'Ojoyoshu' (The Essentials of Salvation). He preached the agreement between the precepts of Buddhism and the Invocation of Buddha's Name, and lectured on unselfishness and mercy at the Imperial Court and for court nobles and ladies, and provincial constables. He restored Saikyo-ji Temple in Sakamoto, Omi Province where Ryogen and Genshin once lived and which Enkan once designated as the main temple for Endonkai (Perfect and Sudden Precepts), and made it as the main temple of the Shinsei School of Tendai Sect.