We report a new strategy for a rapid photoinduced synthesis of monodisperse ligand-coated silver nanoparticles. Such nanoparticles are produced through a very fast reduction of Ag+ by α-aminoalkyl radicals which are first generated from hydrogen abstraction toward an aliphatic amine by the excited triplet state of the 2-substituted thioxanthone series (TX-O-CH2-COO- and TX-S-CH2-COO-). The quantum yield of this prior reaction is tuned by the substituent effect on thioxanthones and leads to a kinetic control of the conversion of Ag+ to Ag(0). Combined with the capping role of a carboxylate function linked to the chromophores, a size regulation of the growing nanoparticles is both promoted and optimized because of a concomitant kinetics adjustment between the photoreduction process and the subsequent functionalization of the nanoparticles. We demonstrate that the optimal adjustment is then obtained with TX-S-CH2-COO-.