In this study, cobalt diselenide (CoSe₂) and the composites of CoSe₂ and reduced graphene oxide (CoSe₂@RGO) were synthesized by a facile hydrothermal reaction of cobalt ions and selenide source with or without graphene oxide for a cost-effective counter electrode material of DSSCs instead of Pt. Electrochemical analyses such as electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) showed that CoSe₂@RGO composites have excellent catalytic activity for reduction of I3- exhibiting synergetic effect between CoSe₂ and RGO. As a consequence, the DSSC using the CoSe₂@RGO CE exhibited the comparable power conversion efficiency (7.01%) to the Pt-based CE device (6.77%).