Unsaturated polyester (UP) resins were filled with up to 3 wt % of bentonites modified with silsesquioxanes (POSS). The effects on mechanical properties and flammability changes of the composites obtained therefrom are presented. The mechanical properties of cured resins were improved; the tensile strength and Charpy impact strength increased by up to 44% and 59%, respectively, compared to unmodified resins. The cured resins had a typical clay nanocomposite structure with layer-like morphology, as observed in scanning electron microscopy (SEM) photomicrographs of brittle fractures, an absence of X-ray diffraction pattern (because of the smectic structure of clays), and exfoliated structure, as evidenced by transmission electron microscopy (TEM) examination. The presence of POSS-modified bentonites in cured UP resins improved their flame resistance, as exemplified by the increase in the limiting oxygen index, from 17.2 for unfilled resin to 25.2 for the resin containing 3 wt % of Wyoming bentonite modified with POSS.