Photocatalytic CO2 reduction has been considered as a promising approach to solve energy and environmental problems. Nevertheless, developing inexpensive photocatalysts with high efficiency and selectivity remains a big challenge. In this study, two Co-based complexes [Co2(L1)Cl2] (1-Co) and [Co(L2)Cl] (2-Co) were synthesized by treating two DPA-based (DPA: dipicolylamine) ligands with Co2+, respectively. Under visible-light irradiation, the performance of 1-Co as a homogeneous photocatalyst for CO2 reduction in aqueous media has been explored by using [Ru(phen)3]2+ as a photosensitizer, and triethylolamine (TEOA) as a sacrificial reductant. 1-Co shows high photocatalytic activity for CO2-to-CO conversion, corresponding to the high TONCO of 2600 and TOFCO of 260 h-1 (TONCO = turnover number for CO; TOFCO = turnover frequency for CO). High selectivity of 97% for CO formation is also achieved. The control experiments catalyzed by 2-Co demonstrated that two Co(II) centers in 1-Co may operate independently and activate one CO2 molecule each. Furthermore, the proposed mechanism of 1-Co for photocatalytic CO2 reduction has been investigated via electrochemical analysis, a series of quenching experiments, and density functional theory calculations.