Cu-activated BaZnOS was studied for the first time as a novel and efficient blue-emitting phosphor. Under the excitation of UV radiation, the phosphor can efficiently give a blue emission centered at 430 nm, corresponding to the transition from conduction band edge to the excited state of Cu 2+ in the BaZnOS host. The maximum emission intensity occurs at 0.08 mol% of the Cu doping content for both photoluminescence (PL) and X-ray excited luminescence. The optimized blue-emitting BaZnOS:Cu phosphor has a larger PL intensity than the well-known green-emitting ZnO:Cu and blue-emitting ZnS:Cu phosphors. The excellent luminescence properties are tightly related to the appropriate direct band gap and the unique crystal structure of BaZnOS host. These results strongly indicate that the Cu-activated BaZnOS is a potential material used as a new high-brightness blue phosphor for UV light-emitting diode and display devices.