Magnesia may refer to:
- a Greek toponym, derived from the tribal name Magnetes
- Magnesia (regional unit), the southeastern area of Thessaly in central Greece
- Magnesia ad Sipylum, a city of Lydia
- Magnesia on the Maeander, an ancient Greek city in Anatolia
- Magnesia Prefecture, a former prefecture of Greece
- Magnesia, a mythical city-state in Plato's Laws
- in chemistry
- Magnesium oxide
- Periclase or magnesia, a natural mineral of magnesium oxide