We propose and experimentally investigate a self-aligned metallic nanocavity that can enhance second harmonic signals from a single KNbO3 nanowire. With pump beams in the spectral proximity of the metallic nanocavity resonance, second harmonic intensity is observed to be >1800 times stronger than that of a typical bare nanowire under the same illumination conditions. By studying spectral features and polarization characteristics, we confirm that the nonlinear enhancement originates from the locally intensified electric field of a surface plasmon-polariton mode. This simple and robust scheme represents a powerful platform to study single nanowire nonlinearity. (C) 2012 Elsevier B.V. All rights reserved.