Constantius may refer to:
- Constantius I, Roman emperor 305–306
- Constantius II, Roman emperor 337–361
- Constantius III, Roman co-emperor in 421
- Constantius (consul 327), consul in 327
- Constantius Gallus (c.325–354), member of the Constantinian dynasty and Caesar of the Roman Empire (351–354); consul from 352 to 354
- Saint Constantius of Perugia (died c. 170), one of the patron saints of Perugia, Italy
- Saint Constantius of Aquino (6th century), bishop of Aquino
- Saint Constantius of Capri, bishop of Capri
- Saint Constantius (Theban Legion)
- Saints Simplicius, Constantius and Victorinus
- Constantius of Lyon, 5th century priest
- Constantius Africanus, (c. 1020–1085), eleventh-century translator of Greek and Islamic medical texts