Hybrid composites of unsaturated polyester resin with the addition of glass microspheres and bentonite modified with a quaternary ammonium salt were obtained. The content of bentonite in composites was 3 wt %, while glass microspheres varied in the range of 5–15 wt %. The effect of the addition of glass microspheres on the rheological and mechanical properties of unsaturated polyester resin composites containing modified bentonite was investigated. On the basis of the obtained results, it was found that the optimum content of glass microspheres in polyester composites at which a clear increase in the value of most mechanical parameters was achieved, is 15 wt %. The Young’s modulus, flexural modulus, tensile strength, and Charpy impact strength increased by 14.7, 38.7, 17.7 and 25%, respectively, compared to the unmodified resin. On the other hand, with relation to the sample containing 3 wt % of bentonite, a significant improvement was achieved only in the case of the flexural modulus and impact strength, amounting to 16.5 and 36.0%, respectively. In addition, composite with 15 wt % of glass microspheres was characterized by a smaller, by about 25%, flexural strength in relation to the unfilled resin and resin with 3 wt % of bentonite. In turn, rheological studies have shown that as the content of glass microspheres increased, there was an increase in viscosity and a decrease in gelation time in relation to the reference sample and the sample with 3 wt % of bentonite.