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MgI2·8H2O is a crystalline powder, white and deliquescent, discoloring in air; soluble in water, alcohol, and ether. This salt has been used in medicine. The anhydrous compound has a molecular weight of 278.1139 g/mol,a melting point of 637°C, a density of 4.43 g/cm3 and a water solubility of 1480 g/l at 18°C. MgI2 has the same type of structure (CdI2) as the chloride and bromide except that the cell parameters differ.
Magnesium iodide is stable at high heat under a hydrogen atmosphere, but decomposes in air at normal temperatures, turning brown due to the release of elemental iodine. When heated in air, it decomposes completely to magnesium oxide, releasing iodine vapor. Magnesium iodide is known to form three hydrates, MgI2·6H2O, MgI2·8H2O and MgI2·10H2O of which the decahydrate is claimed to be the most stable.

Magnesium iodide has few commercial uses but can be used to prepare many compounds for organic synthesis. Usage of magnesium iodide in the “Baylis– Hillman” reaction (an organic reaction of an aldehyde and an α,β-unsaturated electron-withdrawing group catalyzed by MgI2) tends to give vinyl-specific compounds. Another example of the usage of MgI2 as a catalyst involves the reaction of 2,3,4,6-tetra-O-benzyl-D-gluconolactone or 2,3,4,6-tetra-O-benzyl-D-mannono-lactone with MgI2 in EtOH which afforded the respective C-2 epimer.