A new magnesium oxysulfate hydrate phase, 5Mg(OH)2·MgSO4·7H2O was synthesized and structurally characterized. The new phase crystallizes in the form of microsized needles and its crystal structure was solved using X-ray powder diffraction. The crystal packing is made of infinite triple MgO6 chains, intercalated with sulfate groups, water molecules, and hydroxide anions. Using vibrational spectroscopy [infrared (IR) and Raman] and Rietveld refinement, the arrangement of water molecules and hydroxide anions were observed to be statistically distributed and dynamically disordered. The high-temperature behavior and decomposition were studied by thermal analyses and in situ X-ray powder diffraction. The high-temperature measurements reveal the presence of five decomposition stages from the new phase to the production of MgO. After the reactions with atmospheric carbon dioxide were analyzed using micro-Raman spectroscopy, two new carbonate phases were detected on the surface of the material.