Magnesia may refer to:
- Magnesia (hypothetical city), a future colony of Knossos, imagined in Plato's Laws
- Magnesia (regional unit), the southeastern area of Thessaly in central Greece
- Ancient Magnesia, a historical region of Greece with borders differing from the modern regional unit
- Magnesia ad Sipylum, a city of Lydia, now Manisa in Turkey
- Battle of Magnesia, 190 BC, the concluding battle of the Roman–Seleucid War
- Magnesia on the Maeander, an ancient Greek city in Anatolia
- Magnesia Prefecture, a former prefecture of Greece
