A full structure 290-nm ultraviolet light-emitting diode (UV-LED) with a nanoporous n-AlGaN underlayer was fabricated by top via hole formation followed by high-voltage electrochemical etching. The 20 to 120 nm nanopores were prepared in regular doped n-AlGaN by adjusting the etching voltage. The comparison between the Raman spectrum and the photoluminescence wavelength shows that the biaxial stress in the nanoporous material is obviously relaxed. The photoluminescence enhancement was found to be highly dependent on the size of the pores. It not only improves the extraction efficiency of top-emitting transverse-electric (TE)-mode photons but also greatly improves the efficiency of side-emitting transverse-magnetic (TM)-mode photons. This leads to the polarization change of the side-emitting light from $?0.08$ to $?0.242$. The intensity of the electroluminescence was increased by 36.5% at 100 mA, and the efficiency droop at high current was found to decrease from 61% to 31%.