<p>Improving the physical and mechanical properties of clay soils is essential to ensure the stability and durability of structures built on weak soils. This study investigates the synergistic effects of glass fibers and nano-magnesium oxide (nano-MgO) on the geotechnical behavior of clay soil. Mixtures containing varying proportions of glass fibers (0%, 0.5%, 1%, and 1.5%) and nano-MgO (0%, 0.25%, 0.5%, and 1% by dry soil weight) were prepared and tested. Atterberg limits, compaction characteristics, unconfined compressive strength (UCS), elastic modulus (E₅₀), and direct shear tests (DST) were conducted to evaluate the mechanical performance and physical changes of the soil. The results showed that the mixture containing 1.5% glass fibers and 1% nano-MgO yielded the greatest decrease in maximum dry unit weight (MDUW) and increase in optimum moisture content (OMC), with reductions and increases of 11.4% and 46%, respectively, compared to the clay soil. Furthermore, UCS and E₅₀ improved by up to 178% and 143.4%, respectively, compared to the clay soil. Direct shear tests revealed significant enhancements in cohesion and internal friction angle by 140.4% and 112.5%, respectively, indicating improved shear strength parameters. The analysis of the results indicates a strengthening of the soil’s structural network and improved mechanical performance under applied loads. ANOVA Analysis confirmed that both nano-MgO and glass fiber contents, as well as their interaction, had statistically significant effects on UCS, with nano-MgO contributing the most to strength enhancement. These findings confirm that the combined use of glass fibers and nano-MgO substantially improves the physical and mechanical properties of clay soil, offering an effective approach for soil stabilization in geotechnical applications.</p>