Unsaturated polyester resins (UP) display a wide range of applications depending on the proper choice of raw materials and cure conditions. However, some applications are limited to their low-impact resistance, which can be enhanced by the incorporation of modifiers that increase the flexibility of the network. This work aims to incorporate flexible poly(organosiloxane) segments in the UP network as a graft copolymer, as a way of minimizing the low adhesion between the resin and the modifier and enhance its flexibility. Because polyester and poly(organosiloxanes) are immiscible mixtures, the grafting copolymerization during the cure can promote compatibilization between both polymers. Hence, glycidyl methacrylate (GMA) is added to the resin network by radical reaction, as well as 1,3-aminopropyltriethoxysilane (APTS), amino groups of which react with GMA. The addition of a siloxane (1,1,3,3-tetramcthyl-1,3-diethoxydisiloxane) allows the poly(organosiloxane) network to grow and the addition of water guarantees the hydrolysis and polycondensation during the cure. In this way it is possible to increase its flexibility. The grafting copolymerization is evaluated by dynamic mechanical analysis and the impact resistance of pure and modified polyester resins is evaluated by the Izod test. The grafting of a flexible segment in the UP chains enhances the impact resistance of the polyester resin at low-modifier concentration.