Expansive subgrade soils are usually considered as one of the serious geotechnical problems, because they tend to have significantly altered volumes, which may lead to structural damages and stability failure. To improve the strength characteristics of such soil, the present study focuses on examining the feasibility of using guar gum, a natural biopolymer, as an eco-friendly adhesive material. Expansive soil samples were subjected to the incorporation of guar gum in a range of concentrations, from 0.5% to 2% by dry soil weight. A series of lab tests, including the unconfined compressive strength and California Bearing Ratio, as well as the direct shear tests, were carried out to evaluate the strength properties of the treated soil. The results have shown that the guar gum-treated expansive soil featured significantly higher values of both UCS and CBR, which suggests their CBR and shear resistance improvement. According to the findings, the direct shear tests revealed an increase in both cohesion and internal friction angle. This led to the enhancement in shear strength, which was also supported by the significantly tightened shear bonds. The adhesive mechanism is considered to occur within the soil matrix due to the formation of a three-dimensional polymer network, created by intermolecular hydrogen bonding and the cross-linking mechanism. Such a network was found to envelope and bond the soil particles, thus turning the soil into a reinforced composite material, able to withstand the volume changes. The optimal amount of guar gum was identified to be 2%, because such concentration featured the highest improvement in strength characteristics over the untreated soil. The study indicates that guar gum can serve as an efficient and environmentally friendly adhesive material that has the potential to eliminate the negative effects of expansive soils.