Piezoelectric nanogenerators have been intensively developed in terms of their materials and applications; however, only modest structural progress has been made due to limitations in the growth mechanisms of nano-materials. In this work, a piezoelectric nanogenerator based on a nanoforest structure is introduced. Through a metal-assisted chemical etching (mac-etch) process, densely packed vertical nanowires and nanovoids are fabricated. The nanoforest structure serves as a bottom electrode, which increases the interfacial area with a subsequently deposited piezoelectric material, in this case barium titanate (BaTiO3). In the proposed fabrication method, more various piezoelectric materials can be used for a piezoelectric device compared to previously reported methods because the process relies not on the growth mechanism but on the deposition technique. The proposed nanoforest structured nanogenerator produces a 4.2 times enhanced power density compared to the control generator, which uses the same material but has a flat topology. The strong relationship between the enhancement ratio and the nanoforest height is found. Both the experiment and the simulation data present a consistent trend of a gradual increase with a short height and saturation at a tall height.