Methacrylate-functionalized silsesquioxanes (SSQO) were synthesized by the hydrolytic condensation of 3-(methacryloxypropyl) trimethoxysilane (MPTMS) using dimethacrylate monomers as reactive solvent. Resins containing about 40 wt% SSQO displayed a modest increase in viscosity compared with the unmodified methacrylate monomer. This is attributed to a dilution of the reacting medium, which discourages bimolecular reactions that lead to the generation of high-molar mass oligomers and, consequently, prevents undesired increases of viscosity. Substitution of monomer with SSQO reduced the volumetric shrinkage during photopolymerization. The amount of leachable monomer detected in formulations containing SSQO was markedly reduced. This indicates that the unreacted monomer exists mainly as pendant chains instead of free leachable monomer. The presence of different proportions of SSQO in the methacrylate monomer resulted in no apparent change in the flexural modulus and in a modest decrease in the compressive yield stress of the polymerized materials. From results presented in this research, it emerges that methacrylate monomers modified with SSQO are attractive for applications that require reduced volumetric shrinkage and long storage periods, such as light-cured dental restorative composites.