We report a low-cost, facile, and template-free electrochemical method of synthesizing three-dimensional (3D) hollow metallic nanostructures. The 3D nanoporous gold (3D-NPG) nanostructures were synthesized by a galvanic replacement reaction (GRR) using the different reduction potentials of silver and gold; hemispherical silver nanoislands were electrochemically deposited on cathodic substrates by a reverse-pulse potentiodynamic method without templates and then nanoporous gold layer replicated the shape of silver islands during the GRR process in an ultra-dilute electrolyte of gold(III) chloride trihydrate. Finally, the wet etching process of remaining silver resulted in the formation of 3D-NPG. During the GRR process, the application of bias voltage to the cathode decreased the porosity of 3D-NPG in the voltage range of 0.2 to -0.62 V. And the GRR process of silver nanoislands was also applicable to fabrication of the 3D hollow nanostructures of platinum and palladium. The 3D-NPG nanostructures were found to effectively enhance the SERS sensitivity of rhodamine 6G (R6G) molecules with a concentration up to 10(-8) M.