The structure of Li4P2S6 was solved, based on a combination of X-ray powder diffraction data, quantum chemical calculations and solid state nuclear magnetic resonance (NMR). Two-dimensional 31P single quantum/double quantum correlation spectra yielded important constraints regarding the space group symmetry allowing the crystal structure to be solved by the Rietveld method. Li4P2S6 crystallizes in a trigonal space group with a = 10.51452(6) Å; c = 6.59149(8) Å. The structure contains two distinct P2S64- ions in a 2 : 1 ratio: in the first one the two P atoms of the hexahypothiophosphate unit are crystallographically distinct, whereas in the second one they are crystallographically identical.