A simple one-pot approach was developed to obtain nonstoichiometric CuInS2 nanocrystals. Using this approach, both In-rich and Cu-rich CuInS2 nanocrystals could be reliably synthesized by tuning stoichiometric combinations of [Cu]/[In] precursor constituents. By designing Cu-rich CuInS2 heteronanostructures to serve as counter electrodes, quantum-dot-sensitized solar cells (QDSSCs) equipped with In-rich CuInS2 and CdS cosensitizers delivered a power conversion efficiency of 2.37%, which is significantly more efficient than conventional Pt counter electrodes. To the best of our knowledge, this study represents the first report utilizing nonstoichiometric CuInS2 nanocrystals as a photon-harvesting sensitizer comprised of a photoanode and photocathode in QDSSCs; also unique to this report, these nonstoichiometric CuInS2 nanocrystals were formed by simply changing the cationic molar ratios without complicated precursor preparation. Impedance spectroscopy and Tafel polarization indicated that these Cu-rich CuInS2 heteronanostructures had electrocatalytic activities (used for reducing S(2-)/Sn(2-)) that were superior to a Pt catalyst. Moreover, we demonstrated that Cu-rich CuInS2 heteronanostructures were also useful counter electrodes in dye-sensitized solar cells, and this finding revealed a promising conversion efficiency of 6.11%, which was ∼96% of the efficiency in a cell with a Pt-based counter electrode (6.32%).