A study was conducted to demonstrate the application of indium tin oxide (ITO) nanocolumns as a conductive antireflection (AR) coating layer for GaAs solar cells. Characteristic ITO nanocolumns, prepared by glancing-angle deposition with an incident nitrogen flux offered omni-directional and broad-band AR properties for both s- and p-polarizations. Calculations based on a rigorous coupled-wave analysis (RCWA) method indicated that the superior AR characteristics arose from the tapered column profiles, which collectively functioned as a graded-refractive-index layer. It was observed that the conversion efficiency of the GaAs solar cell with the nanocolumn AR layer increases by 28% as compared to a cell without any AR treatment. It was also observed that around 42% enhancement was achieved for photocurrents generated at wavelengths that were transparent to the window layer.