Covalent organic frameworks (COF) display a unique combination of chemical tunability, structural diversity, and porosity. Recent efforts in the field are directed at using such frameworks as tunable scaffolds for chemical reactions. Owing to their built-in photoactivity and nanoscale regularity, COFs have emerged as viable platforms for mimicking natural photosynthesis. While previous approaches have focused on platinum nanoparticle or molecular co-catalysts physisorbed in the COF pores, controlling interfacial charge transfer through close COF-co-catalyst contact remains an open challenge. Here, we present a covalently bound COF-co-catalyst hybrid based on an earth-abundant azide-functionalized cobaloxime hydrogen evolution catalyst immobilized on a hydrazone-linked COF-42 backbone. This single-site heterogeneous catalyst shows improved and prolonged photocatalytic activity with respect to the corresponding physisorbed systems. Advanced solid-state NMR and quantum chemical methods reveal details of the improved photocatalytic activity and the structural composition of the involved active site. We found that a genuine interaction between the COF backbone and the cobaloxime facilitates charge transfer and re-coordination of the co-catalyst during the photoreaction, thereby improving the reactivity and hindering degradation of the catalyst. This study highlights the importance of engineering the COF-co-catalyst interface and at the same time provides pertinent design principles for improved polymeric photocatalysts in general.