We have found that an organic molecule as simple as p-anisaldehyde efficiently catalyzes the intermolecular atom-transfer radical addition (ATRA) of a variety of haloalkanes onto olefins, one of the fundamental carbon–carbon bond-forming transformations in organic chemistry. The reaction requires exceptionally mild reaction conditions to proceed, as it occurs at ambient temperature and under illumination by a readily available fluorescent light bulb. Initial investigations support a mechanism whereby the aldehydic catalyst photochemically generates the reactive radical species by sensitization of the organic halides by an energy-transfer pathway.