Developing high-performance, low-cost electrocatalysts for the reduction reaction at the cathode of dye-sensitized solar cells (DSCs) is important for their commercialization. Here, we introduce the concept of redox catalysis for improving counter-electrode (CE) kinetics in DSCs for the first time. As a proof-of-concept, we employ a catalytic system based on mesoporous indium-tin oxide electrodes modified by redox-active Os complex molecules (Os@ITO), which are fabricated by using a convenient method similar to that used for sensitizing the DSC photoanode. The adsorbed molecular catalyst displays high catalytic activity for the reduction of I-3(-) species in aqueous solution, and dramatically increased fill factor (FF) in photovoltaic measurements. Analysis of DSC current-voltage characteristics under simulated sunlight, together with current-overpotential characteristics and electrochemical impedance spectra of various CEs, reveals that the photovoltaic performance enhancement brought about by anchoring the Os complex to the mesoporous ITO electrode can be attributed to a roughly ten-fold enhancement in exchange current density for the I- /I-3(-) redox reaction. It is anticipated that these results will stimulate further research into redox catalysis as a new CE concept for DSCs.