A novel Li2NaPS4 crystal was successfully prepared from tetragonal Na3PS4 via Na+/Li+ ion-exchange in an acetonitrile solution of lithium bis(fluorosulfonyl)amide at room temperature. Its crystal structure was analyzed by powder X-ray diffraction. The Li2NaPS4 crystal maintained the basic anion framework of tetragonal Na3PS4 (P (4) over bar 21c). The chemical environment of PS43- units in the ion-exchanged product was similar to that in the Li3PS4 crystal. Ion-exchange synthesis is a promising approach to develop novel sulfide-based materials.