The widespread implementation of high-energy-density lithium metal batteries has long been fettered by lithium dendrite-related failure. Here we report a new strategy to address the issue of dendrite growth by a polyimide-coating layer with vertical nanoscale channels of high aspect ratio. Smooth, granular lithium metal was deposited on the modified electrode instead of typical filamentary growths. In a comparison with the bare planar electrode, the modified electrode achieved greatly enhanced Coulombic efficiency and longer cycle life. Homogeneous Li+ flux distribution above the modified electrode from the nanochannel confinement can account for a uniform Li nucleation and a nondendrite growth. We also demonstrated that the polyimide coating with microscale pores loses the confinement effects and fails to suppress lithium dendrites. This strategy of spatially defined lithium growth in vertical-aligned nanochannels provides a novel approach and a significant step toward stabilizing Li metal anodes.