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