In this work we study the effects of TiO2, Co3O4, and NiO nanoparticle additives on the properties of agarose polymer electrolytes. We characterize the crystallinity, surface morphology, ion concentration, ionic conductivity, and pore filling capability of the modified electrolytes using a variety of experimental methods. Solid-state dye-sensitized solar cells are also fabricated using the modified agarose electrolytes, and their photovoltaic and electrochemical performances are examined. The DSSC based on Co3O4-modified agarose electrolyte shows the best device performance including a power conversion efficiency of 2.11% and good device stability after 400 h of testing.