High power Na-ion rechargeable batteries have attracted much interest recently. In particular, the development of nanostructured electrode materials is essentially required, because the large surface area and short Na-ion diffusion length could provide the high power density. In this paper, we report on the fabrication of single-crystalline Na0.44MnO2 nanowire, and the application to Na-ion rechargeable batteries. The single phase Na0.44MnO2 was successfully synthesized by the hydrothermal method. The SEM and TEM experiments proved that hydrothermally synthesized Na0.44MnO2 has single-crystalline nanowire morphology. The single-crystalline Na0.44MnO2 nanowire electrode in Na-ion batteries showed both the excellent cycle stability and high-charge/discharge-rate capability.