Herein, a new red-emitting phosphor, Na3TaF8:Mn4+, was synthesized by non-equivalent doping of Mn4+ under mild condition. Its crystal structure and morphology were characterized by the powder X-ray diffraction (XRD), scanning electron microscopy (SEM). This sample is of single phase with monoclinic crystal structure. Mn4+ might occupy the site of Ta5+ in Na3TaF8, being located at the center of uniquely tetragonal anti-prism coordinated by eight F-. The as-prepared Na3TaF8:Mn4+ has sharp red emissions with higher color purity under the blue light excitation. The luminescent behavior of Mn4+ in such host is similar with that in some hexafluorides with disordered octahedral structure. Intense red light can be observed from the single red LED based on Na3TaF8:Mn4+. These results show this phosphor to be a potentially red component for LED backlighting.