Gelasius is a masculine given name, from Greek (Gelásios), from Ancient Greek (gélasis, “laughing”). It may refer to:
- Gelasius of Caesarea (died 395), bishop of Caesarea
- Pope Gelasius I (died 496)
- Gelasius of Nilopolis (fifth century), Egyptian Christian abbot
- Gelasius of Cyzicus (fifth century), ecclesiastical writer
- Pope Gelasius II (died 1119)
- Gelasius, Archbishop of Armagh (1137 to 1174)
- Gelasius Ó Cuileanáin, Cistercian Abbot of Boyle, Ireland
- Nikollë Gazulli (1895–1946), who wrote under the pseudonym Gelasius