We report herein the investigation of TaON nanoparticles incorporating a reduced graphene oxide (RGO) nanocomposite as a counter electrode for application in Co(bpy)3(3+/2+) (bpy = 2,2'-bipyridine)-mediated dye-sensitized solar cells (DSSCs). The RGO-TaON nanocomposite has been prepared by mixing graphene oxide (GO) and presynthesized TaON nanoparticles in ethanol/water followed by the facile hydrazine hydrate reduction of GO to RGO. Compared with RGO or TaON alone, the RGO-TaON nanocomposite shows a much higher electrocatalytic activity for the reduction of Co(bpy)3(3+) species owing to synergistic effects, resulting in significantly improved solar-cell performance when it is applied as the counter electrode in DSSCs. An efficiency of 7.65% for the DSSC with the RGO-TaON counter electrode is obtained, competing with the efficiency produced by the Pt counter electrode; additionally, the former exhibits a much better electrochemical stability than the latter in a Co(bpy)3(3+/2+) acetonitrile solution.