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