氧化镁（化学式：MgO）也称苦土，是镁的氧化物，一种离子化合物；常温下为白色固体；氧化镁以方镁石的形式存在于自然界中。

氧化镁可由煅烧碱式碳酸镁（碳酸镁类）或氢氧化镁制得；煅烧温度在400－900℃制成轻质氧化镁，在900℃以上制成重质氧化镁。

也可用二氧化碳当氧化剂制造。

Magnesium oxide (MgO), or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium (see also oxide). It has an empirical formula of MgO and consists of a lattice of Mg ions and O ions held together by ionic bonding. Magnesium hydroxide forms in the presence of water (MgO + H₂O → Mg(OH)₂), but it can be reversed by heating it to separate moisture.