Surface-modified titania nanoparticles were prepared and their suitability as additives for the long-term UV-photoprotection of polymer materials was investigated. It was demonstrated that the functionalization of nano-TiO2 with different fatty acid and phosphonate-based grafting reagents has a beneficial effect on the long-term stability of polyester composites under accelerated weathering conditions. These results are of considerable interest for the development of robust organic–inorganic hybrid systems that can be utilized as transparent but UV-resistant protective layers for outdoor applications.