Boranates (tetrahydroborates) are studied intensively because of their potential use as hydrogen storage materials. In this Article, we present a first-principles study of α-Mg(BH4)2 at the level of density functional theory. We optimize the complex structure of α-Mg(BH4)2, starting from the experimental crystal structures with 30 formula units per unit cell. From total energy calculations, incorporating vibrational contributions at finite temperature, we show that the hydrogen desorption reaction α-Mg(BH4)2 → MgB2 + 4H2 has a reaction enthalpy of 38 kJ/mol H2 at room temperature. This makes Mg(BH4)2 an interesting candidate as a hydrogen storage material.