The abiotic synthesis of organic compounds from CO₂ and water under prebiotic conditions is a fundamental yet unresolved challenge in understanding the origins of life. Here we demonstrate a radical-mediated pathway for reducing CO₂ to C₁‒C₆ hydrocarbons and oxygenates driven solely by ultraviolet (UV) irradiation of water, mimicking early Earth environments. Using electron paramagnetic resonance (EPR), ¹⁷O/¹³C isotope labeling, and femtosecond transient absorption, we identify ionized water-derived radicals (H₂O^•+, ^•OH, e⁻_aq, ^•H) as the key redox mediators. e⁻_aq acts as a super-reductant (−2.9 V) to activate CO₂ into CO₂^•⁻, while ^•H enables sequential hydrogenation. Critically, oxidative radicals (H₂O^•+ and ^•OH) recycle recalcitrant oxygenates (formate and oxalate) back into active CO₂^•⁻, sustaining a dynamic radical network. This process generates a diverse library of organic compounds, including methane, ethylene, and C₆ dimethyl succinate, via radical assembly mechanisms spanning hydrogen-atom transfer (HAT), self-coupling, and cross-coupling. By integrating experimental validation with prebiotic simulations (formate-mediated redox modulation), we resolve the paradox of inert CO₂/H₂ activation in primordial environments and establish water not merely as a solvent but as a reactive matrix directing abiotic organic synthesis.