Abstract: NADH is a key redox mediator in biology and biocatalysis, yet its catalytic use in non-enzymatic synthetic chemistry remains largely unexplored. Here, we show that NADH can act as a substoichiometric reductive quencher in red-light photoredox catalysis, enabling a redox-neutral C(sp2)–H alkylation of acceptor heterocycles. This strategy reduces waste and leads to excellent yields and selectivity by suppressing overreduction. The reaction proceeds in air, under mild, aqueous-compatible conditions and operates in biorelevant media, establishing NADH as a cofactor for artificial red-light photoredox catalysis.