Due to clay soil's low bearing capacity and shear strength, mechanical and chemical stability are required. The objective of this study was to increase the California bearing ratio (CBR) value of clay as a subgrade material by utilizing gypsum waste and determining the percentage of gypsum waste added to the soil to produce an optimal CBR value. According to the USCS classification, the soil sample used in this study was a CL soil sample (Clay-Low Plasticity). Gypsum waste is used as a stabilizer in varying proportions of 10%, 15%, 20%, and 25%, based on the weight of the soil sample. The untreated soil's maximum dry density (MDD) was 1.53%, and the maximum water content was 23.91%. The addition of 25% gypsum resulted in a maximum dry density of 1.571 g/cm3 and an optimum water content of 18.95%. Furthermore, 25% gypsum-treated soil increased the value of both soaked and unsoaked CBR. The addition of 17% gypsum resulted in the best CBR value. The Unconfined Compression Test results revealed that the highest compressive strength (qu) was obtained at 21 days cured and 17% gypsum, with an MDD value of 1.593 kg/cm2.