Natural weak cohesive subgrade soils are considered low quality foundations for pavement structures that may lead to significant pavement distresses. Several solutions can be proposed to enhance pavement behavior under cyclic loads such as stabilizing clays with cement or lime, removing and replacing weak material, or increasing the base thickness to reduce the stresses on subgrade. Despite the effectiveness of using additives like lime and cement for subgrade stabilization, cement and lime are considered environmentally unfriendly owing to the generated emission during their production. Recently, several researchers have shown the potential of using biopolymers as alternative for traditional additives. This study investigates the feasibility of using sodium alginate biopolymer for treating weak cohesive subgrades particularly under repeated traffic loads for pavement construction applications. Two different soils (clay and silt), representing a typical range of cohesive soils in the delta region in Egypt, were investigated. The sodium alginate was added to the cohesive soils in the range of 0% to 6% using two different mixing methods (wet and dry). The experimental program included basic engineering tests, resilient modulus, and unconfined compressive strength tests, along with other chemical tests such as Fourier transform infrared spectroscopy (FTIR) and thermal gravimetric analysis (TGA). In addition, scanning electronic microscope (SEM) images were performed for both untreated and treated soils at different alginate contents. Results showed that the resilient modulus of the treated subgrade soils generally increased with the increase in sodium alginate content up to an optimum value of 2% and 4% for clayey and silty soils, respectively. This significant improvement was reached in the first 4 days of curing with continuous gain of strength up to 28 days. This increase in resilient modulus was found to be dependent on the soil type, alginate concentration, curing time, and treatment method. The microlevel tests, TGA, SEM, and FTIR, confirmed the presence of the cementation polymer due to sodium alginate that increased the bond and cross-linking between particles, and consequently an increase in the soil stiffness and strength occurred.