Adenovirus (HAdV), a double-stranded DNA virus, exhibits resistance to direct inactivation by UVC and solar UVB radiation in water, owing to its ability to repair UV-induced genome damage using host cellular machinery. In the presence of dissolved organic matter (DOM), however, previous studies have reported unexpectedly rapid sunlight-mediated inactivation, even after accounting for indirect effects mediated by photochemically produced reactive intermediates (PPRIs). Here, we hypothesized that a synergistic interaction between direct and indirect inactivation pathways compromises HAdV2's genome repair capacity, thereby enhancing its overall susceptibility to inactivation. First, we demonstrated that pre-exposure to singlet oxygen (1 O 2), a key PPRI, significantly increased HAdV2's susceptibility to subsequent direct UV inactivation. Using host cells with differing genome repair capacities, we further showed that simultaneous exposure to direct and indirect inactivation pathways reduced HAdV2's genome repair efficiency compared to direct UV alone. Finally, although indirect inactivation caused minimal DNA damage, it impaired the DNA replication efficiency of host cells, likely due to oxidative damage to viral proteins involved in transcription-coupled repair. These findings highlight a critical interplay between direct and indirect inactivation pathways and offer new insights that can aid in optimizing light-based disinfection strategies to enhance the efficacy of water treatment processes targeting adenovirus-contaminated sources.