Readily available and facilely tunable pyridine N-oxides have been developed as effective photoinduced hydrogen-atom-transfer (HAT) catalysts for site-selective C–H functionalizations of a broad range of C–H substrates, including unactivated alkanes. Pyridine N-oxide radicals, catalytically generated from N-oxides by photoredox catalyzed single-electron oxidation, are the key intermediates that enable an effective HAT process for carbon radical generation to achieve alkylation, amination, azidation, allylation, and cyanation. Additionally, the fine-tuning of reactivity and selectivity of pyridine N-oxides through operationally simple structural modification was investigated and showed promising capability for selective functionalization of unactivated C(sp³)–H bonds.