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

• Magnesium oxide
  • Periclase or magnesia, a natural mineral of magnesium oxide

• Magnesian limestone (disambiguation)
• Magnesium
• Milk of magnesia, a suspension of magnesium hydroxide