Gelasius may refer to:
- Pope Gelasius I (died 496)
- Pope Gelasius II (died 1119)
- Gelasius of Cyzicus, ecclesiastical writer
- Gelasius of Caesarea (died 395), bishop of Caesarea
- Gelasius, Archbishop of Armagh (1137 to 1174)
- Gelasius O'Cullenan, Cistercian Abbot of Boyle, Ireland