A nanohybrid comprises of cobalt-nickel selenide (Co0.50Ni0.50Se) nanoparticles dispersed on graphene nanosheets (GN) has been synthesized by a simple one-step hydrothermal method to use as a counter electrode for dye sensitized solar cell (DSSC). The structural changes and the corresponding electrocatalytic properties in terms of cathodic peak current density and charge transfer resistance at the electrode/electrolyte interface for Co0.5Ni0.5Se/GN nanohybrid with respect to the mass ratio between Co0.5Ni0.5Se nanoparticles and graphene nanosheets have been investigated. Co0.5Ni0.5Se/GN0.50 exhibits higher electrocatalytic activity and lower charge transfer resistance than other Co0.5Ni0.5Se/GN nanohybrids and std. platinum (Pt) counter electrodes. Finally, the DSSC fabricated with Co0.5Ni0.5Se/GN0.50 as the counter electrode delivered photo-conversion efficiency (PCE) of 9.42%, which is higher than that of DSSC fabricated with Co0.5Ni0.5Se (7.59%) and std. Pt (7.68%) counter electrodes. This improved electrochemical performance and higher PCE incited by the synergistic effect of Co and Ni in Co0.5Ni0.5Se nanoparticles, as well as Co0.5Ni0.5Se and graphene, could make Co0.5Ni0.5Se/GN0.50 nanohybrid as a promising low-cost Pt-free counter electrode for DSSC.