We report the synthesis of a three-dimensional graphene (3DG)-TiO2 nanocomposite by covalently attaching P25 TiO2 nanoparticles onto pristine 3DG through a perfluorophenyl azide-mediated coupling reaction. The TiO2 nanoparticles were robustly attached on the 3DG surface, with minimal particle agglomeration. In photocatalytic CO2 reduction, the 3DG-TiO2 nanocomposite demonstrated excellent activity, about 11 times higher than that of the P25 TiO2 nanoparticles. The enhanced activity can be partially attributed to the highly dispersed state of the P25 TiO2 nanoparticles on the 3DG substrate. This 3DG-based system offers a new platform for fabricating photocatalytic materials with enhanced activities.