The crystal structure of anhydrous δ-D-mannitol (C6H 14O6) was solved from high-resolution synchrotron X-ray powder diffraction data collected on a mixture containing 20% and 80% w/w of β- and δ-D-mannitol, respectively. The direct space simulated annealing program PSSP, and Rietveld analysis employing GSAS were used to determine and refine the structure. The polymorph has monoclinic symmetry, space group P21 with a = 5.08941(5) Å, b=18.2504(2) Å, c = 4.91702(5) Å, and β = 118.303(2)°. There is one molecule in the irreducible volume of the unit cell. The pattern of hydrogen bonding is significantly different than the previously known α and β forms.