We report a Michael-type cyanation reaction of coumarins by using CO₂ as a catalyst. The delivery of the nucleophilic cyanide was realized by catalytic amounts of CO₂, which forms cyanoformate and bicarbonate in the presence of water. Under ambient conditions, CO₂-catalyzed reactions afforded high chemo- and diastereoselectivity of β-nitrile carbonyls, whereas only low reactivities were observed under argon or N₂. Computational and experimental data suggest the catalytic role of CO₂, which functions as a Lewis acid, and a protecting group to mask the reactivity of the product, suppressing byproducts and polymerization. The utility of this convenient method was demonstrated by preparing biologically relevant heterocyclic compounds with ease.