Heterogeneous catalysts comprising single-atom (SA) catalytic sites represent a distinct class of catalysts that offer several advantages over conventional metal or metal oxide particle catalysts. In addition to improved atom economy, the active sites of single-atom catalysts (SACs) possess distinct geometries, electronic structures, and energetics that can result in superior performance in terms of activity and selectivity. This unique combination of attributes sets SACs apart from traditional catalysts and highlights their potential for advancing catalytic processes. Formation of effective SA catalytic sites holds also great promise in the field of heterogeneous photocatalysis where the deposition of additional catalysts (so called "cocatalysts") onto the surface of light-absorbing photocatalysts is often mandatory in order to improve the extraction of photogenerated charges (i.e., electrons and holes), enhance the rate of their reaction with reactants, and thus to increase the quantum yield of the photocatalytic processes which is typically only few percent in the absence of co-catalysts. A particular advantage of SACs in photocatalysis is their very low effective cross-section for light absorption as compared to metal or metal oxide catalysts with a fully developed long-range electronic structure, which makes it easier to avoid the parasitic light absorption by the co-catalyst that would otherwise block the excitation of the light absorber and hamper the overall photocatalytic activity. Recently, several examples of various photocatalysts (e.g., TiO2, CdS, BiVO4, polymeric carbon nitride, carbon quantum dots) modified with SACs comprising different metal (e.g., Pt, Ir, Rh, Fe, Co, Ni, Cu, Sn, In) atoms or ions have been demonstrated for various applications, including degradation of harmful organic compounds, hydrogen evolution, water oxidation to dioxygen, or useful selective light-driven conversions, such as dioxygen reduction to H2O2, selective oxidations of alcohols, benzene, benzylamine, and NOx, or C-C coupling reactions. However, our knowledge on the mechanism of photocatalysis enhancement at SAC-modified photocatalysts is still rather limited, which makes the rational design of high-performance photocatalysts based on SACs very challenging.