A water-promoted DABCO-organocatalyzed hydrodecarboxylation of conjugated carboxylic acids has been developed, providing the corresponding decarboxylation products in good to excellent yields. This metal-, photocatalyst-, and oxidant-free protocol features high functional group tolerance, wide substrate scope, gram-scale synthesis, and operational simplicity under open air conditions at room temperature. A trace amount of water as the hydrogen source for the successful conversion and the origin of the chemoselectivity are demonstrated through a series of control experiments.