<p>Radiation chemistry studies the chemical effects of ionizing radiation on matter. It has found broad applications across diverse scientific, medical and industrial fields. Recently, radiation-mediated transformations have emerged as valuable tools for activating inert molecules or enabling late-stage functionalization. Nevertheless, these applications are often limited by poor chemoselectivity arising from the diverse radical species generated by radiation. Inspired by advances in radical tuning within metallaphotoredox chemistry, here we show that a nickel catalytic system may precisely regulate these radical species, thereby enabling the chemoselectivity in radiation chemistry. When applied to C−H activation and arylation, this approach improves the yield of the desired product from less than 5% to up to 82%, suggesting the potential to overcome the long-standing challenge of poor chemoselectivity in radiation-driven transformations. Of note, this method enables selective C−H arylation without the need for photosensitizers and remains compatible with photoactive substrates. In addition, with the use of chiral ligands, we demonstrate enantioselective C(<i>sp</i>³)−H arylation under ionizing radiation, achieving enantiomeric excesses of up to 90%.</p>