Geotechnical investigations are one of the most crucial parts of the construction to save the time and effort spent on the projects. Particularly, in the case of the clays with high plasticity are encountered, the analysis of bearing capacity and settlement of structures such as buildings, roads, and dams governs the design. The possible solutions can be leaving the land, reaching a strong soil bypassing the weak one, removing the weak soil by replacing it with a higher strength of the material, and applying soil improvement methods. Glass fibers are one of the synthetic additives which may be used to address improving the weak soil properties. In this study, the effect of alkaline-resistant glass fiber additive on the undrained shear strength of the clayey soils was investigated. Fibers with distinct lengths were added to the natural soil at increasing rates, and the mixtures were prepared in the optimum water content. The prepared mixtures were compacted in a compaction mold with a diameter of 50 mm and a height of 100 mm with standard energy. Unconfined compression test (UC) were conducted on a total of 210 cylindrical samples, 10 of natural clay and 200 of fiber-reinforced. The maximum increase was at 0.5% of a fiber blend of 40 mm length. As a result of 210 UC tests, it was observed that the alkaline-resistant glass fiber additive increased the shear strength of the clay soil.