We report the synthesis of PtSn alloy/reduce graphene oxide (RGO) nanohybrid materials and their application as efficient counter electrodes (CEs) in dye-sensitized solar cells (DSCs). Well-dispersed Pt_1-xSn_x (1 ≥ x ≥ 0) alloy nanoparticles (NPs) were stabilized with RGO after a co-reduction of metal precursor ions and graphene oxide via a dry plasma reduction under an atmospheric pressure and close to room temperature. The nanohybrids exhibit a 3D network structure of RGO with high NP loading on the RGO surface. The NP size is in a range of 1 – 4 nm, with a mean size of 2 nm. The structure of the PtSn alloy/RGO nanohybrid was confirmed by X-ray diffraction spectroscopy and HAADF-STEM measurements. The chemical composition of a binary alloy can be adjusted by varying the volume ratio of the Pt to Sn precursors, as confirmed by an XPS analysis. We optimized the nanohybrid structure for high catalytic activity toward a reduction of triiodide ions. The highest electrocatalytic performance, which corresponds to the lowest charge transfer resistance of 1.12 Ω, is achieved with the Pt_0.9Sn_0.1 NPs/RGO nanohybrid. The application of the optimized Pt_0.9Sn_0.1 NPs/RGO nanohybrid as an alternative CE for DSCs results in an increase of the efficiency by 15% over that of a Pt/RGO-based DSC and an increase of 46% over the efficiency of a Pt-free device. These results suggest that tailoring the electronic structure of the alloy can serve to enhance the catalytic activity of a nanohybrid CE and, in turn, be utilized to formulate high-efficiency DSCs.