Magnesia may refer to:

Chemistry and geology

  • Magnesium oxide
  • Periclase or magnesia, a natural mineral of magnesium oxide
  • Magnesian limestone (disambiguation)
  • Magnesium
  • Milk of magnesia, a suspension of magnesium hydroxide

Geography

  • Magnesia (hypothetical city), a future colony of Knossos, imagined in Plato's Laws
  • Magnesia (regional unit), the southeastern area of Thessaly in central Greece
  • Magnesia (constituency), electoral district
  • 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