Orsisius was a fourth-century Egyptian monk, writer, and abbot who lived as a disciple of Pachomius the Great. Following the death of his mentor on the Island of Tabenna, he initially declined the leadership of the community, choosing instead to support Theodore. However, after Theodore passed away around 380, Orsisius accepted the role of hegumen upon the advice of Athanasius. He is recognized as a saint within various Christian traditions, including Catholic, Orthodox, and Oriental branches.