An alkyl-functionalized hyperbranched polymer, HBP(OH)-C16, was synthesized by partial modification with fatty acid of an aromatic-aliphatic OH-terminated hyperbranched polyester HBP-OH. This product was used as additive in the cationic photopolymerization of an epoxy resin. The alkyl-modified polyester takes part in the photopolymerization process thanks to the residual OH groups by means of chain-transfer reactions. An increase of the epoxy conversion is observed by increasing the amount of the HBP additive in the photocurable resin with a modification of the bulk properties of the final ultraviolet-cured films. The presence of HBP(OH)-C16 induces an increase in glass transition temperature, thermal stability, and solvent resistance. Moreover the surface properties of the films are modified achieving highly hydrophobic surfaces in the presence of even very low amounts of HBP(OH)-C16.