Direct and sensitized excitation of 3-(N-phthalimido)adamantane-1-carboxylic acid (1) leads to the population of the triplet state that, in the presence of a base, decarboxylates, giving N-(1-adamantyl)phthalimide (2) cleanly and efficiently (Φ = 0.11). The radical initially formed by decarboxylation adds regiospecifically to electron deficient alkenes, whereas radical addition was not observed for electron rich alkenes. The radical addition can also be applied to molecules not bearing adamantanes wherein the electron donor (carboxylate) and the acceptor (phthalimide) are separated by a rigid spacer. The photodecarboxylation induced radical addition of phthalimide derivative 1 to alkenes takes place in good to excellent yields and represents a mild and efficient method for C-C bond formation.