We report on a simple and efficient process to enhance the performance of perovskite solar cells by using ITO electrodes with a nanoporous surface formed by wet-etching of self-agglomerated Ag nanoparticles. Effective removal of the Ag nanoparticles embedded in the surface of the ITO electrodes result in a nanoporous structure without changing the ITO's sheet resistance (10.17 Omega/square) and optical transmittance (89.08%) at a 550 nm wavelength. Examinations with a scanning electron microscope, a transmission electron microscope, and two-dimensional porous mapping show that the nanoporous ITO surface has an increased contact area with the electron transport layer, which enhanced the carrier extraction efficiency of the perovskite solar cells. Compare to perovskite solar cells fabricated on typical ITO with a flat surface morphology, the perovskite solar cells fabricated on the nanoporous-surface ITO show a higher fill factor of 81.1% and a power conversion efficiency of 20.1%. These results indicate that modified ITO surfaces with nano-scale porosity provide a simple and efficient method to improve the power conversion efficiency of perovskite solar cells without a complicated process.