Sulfide-based Na-ion conductors are promising candidates as solid-state electrolytes (SSEs) for fabrication of solid-state Na-ion batteries (NIBs) because of their high ionic conductivities and low grain boundary resistance. Currently, most of the sulfide-based Na-ion conductors with high conductivities are focused on Na3PS4 phases and its derivatives. It is desirable to develop Na-ion conductors with new composition and crystal structure to achieve superior ionic conductivities. Here we report a new quaternary Na-ion conductor, Na10.8Sn1.9PS11.8, exhibiting a high ionic conductivity of 0.67 mS cm−1 at 25 °C. This high ionic conductivity originates from the presence of a large number of intrinsic Na-vacancies and three-dimensional Na-ion conduction pathways, which has been confirmed by single-crystal X-ray diffraction and first-principles calculations. The Na10.8Sn1.9PS11.8 phase is further evaluated as an electrolyte in a Na-Sn alloy/TiS2 battery, demonstrating its potential application in all-solid-state NIBs.