Well-crystallized Cu2ZnSnS4 (CZTS) nanoparticles contain ultrasmall nanocrystals (similar to 10 nm) have been grown directly on three-dimensional (3D) transparent porous reduced graphene oxide (rGO) thin films by a facile and scalable solution-based strategy. Few-layer rGO prepared by modified Hummers' method was used to fabricate hierarchical ultraporous 3D rGO thin films (3DGTFs) with high transmittance (> 75% for 200-nm thick). Single-phase kesterite CZTS nanocrystalline particles were grown uniformly on the surface active sites within the 3D rGO network by hydrothermal method. The as-prepared CZTS/rGO composite thin films exhibited excellent electrocatalytic ability by taking advantages of the high conductivity and high surface area of 3DGTFs and the high catalytic activity of CZTS nanoparticles. As expected, the composite thin films demonstrate more than one order of magnitude lower in electrical resistivity and in charge transfer resistance than the individual CZTS thin films. The conversion efficiency of dye-sensitized solar cells using CZTS/rGO thin films as the counter electrode (CE) approached 6.12%, comparable to that using Pt CE (6.45%) and superior to those using individual CZTS CE (1.07%) and rGO CE (0.18%).