Abstract The C−C coupling of methane (CH 4 ) and carbon dioxide (CO 2 ) to generate acetic acid (CH 3 COOH) represents a highly atom‐efficient chemical conversion, fostering the comprehensive utilization of greenhouse gases. However, the inherent thermodynamic stability and kinetic inertness of CH 4 and CO 2 present obstacles to achieving efficient and selective conversion at room temperature. Our study reveals that hydroxyl radicals (⋅OH) and hydrated electrons (e aq − ) produced by water radiolysis can effectively activate CH 4 and CO 2 , yielding methyl radicals (⋅CH 3 ) and carbon dioxide radical anions(⋅CO 2 − ) that facilitate the production of CH 3 COOH at ambient temperature. The introduction of radiation‐synthesized CuO‐anchored TiO 2 bifunctional catalyst could further enhance reaction efficiency and selectivity remarkably by boosting radiation absorption and radical stability, resulting in a concentration of 7.1 mmol ⋅ L −1 of CH 3 COOH with near‐unity selectivity (>95 %). These findings offer valuable insights for catalyst design and implementation in radiation‐induced chemical conversion.