The increasing demand for lightweight, high-strength composite materials in aerospace, automotive, and structural applications has intensified research into polymer composites. To address the challenge of enhancing mechanical properties while maintaining weight efficiency, this study investigates the effect of silica nanoparticles (SNiPs) on the flexural and compressive strength of sandwich composites. These composites are constructed from fiber-reinforced carbon, para-aramid, glass, and polyurethane (PU) foam. Composite specimens with varying concentrations of SNiPs were produced using a hand layup process with synthetic fiber-reinforced polymer glass fibers (GFRP), carbon fibers (CFRP), para-aramid fibers (KFRP), urethane foam, and epoxy glue. Fabricated using a quasi-static stacking sequence technique, the composites feature a three-layer sandwich matrix(0 K 1 / 45° C 1 / 45° G 1/PU/ 45° G 1 / 45° C 1 /0 K 1). Flexural and compressive strength tests were conducted to assess their mechanical behavior. The results show that incorporating SNiPs into polymer hybrid sandwich composites significantly enhances both flexural and compressive strength. This improvement is attributed to the high aspect ratio and surface properties of SNiPs, which facilitate effective stress transfer between the reinforcing fibers and the polymer matrix. The flexural strengths achieved with 0, 2, 4, and 6 wt.% SNiPs were measured as 52.583, 58.713, 64.108, and 61.397 MPa, respectively. Similarly, the compressive strengths were measured as 2.207, 2.813, 3.528, and 3.182 MPa for the respective weight percentages.