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Stabilization of sulfate-rich expansive subgrade soils is a persistent cause of concern for transportation infrastructure engineers and practitioners. The application of traditional calcium-based stabilizers is generally not recommended for treating such soils because of the formation of deleterious reaction products such as ettringite. Sulfate-induced heaving causes severe structural damage to pavements and accounts for enormous expenditure from routine maintenance and rehabilitation activities. A research study was undertaken to evaluate the feasibility of using a metakaolin-based geopolymer (GP) for the treatment of sulfate-rich expansive soil. Laboratory studies were conducted on natural soil and artificially sulfate-rich soils, when treated with either lime or GP, to evaluate and compare the improvements in the engineering properties, including unconfined compressive strength, swelling and shrinkage, and resilient moduli characteristics over different curing periods. Microstructural studies, such as field emission scanning electron microscopy and X-ray diffraction, were performed on treated soils to detect the formation of reaction products. The engineering studies indicate that GP treatment enhanced strength and resilient moduli while suppressing ettringite formation and the associated swell–shrink potential of the treated soils. The microstructural studies showed that GP gels contribute to the improvement of these engineering properties through the formation of a uniform geopolymer matrix. In addition, the absence of a calcium source suppressed the formation of ettringite in the GP-treated soils. Overall, the findings indicate that GPs could be used as a potential alternative to existing traditional stabilizers for treating sulfate-rich expansive soils.