The growing demand for urban development requires the construction of infrastructure on weak soils. Chemical stabilization has emerged as an effective technique to enhance soil performance, offering a cost-efficient and sustainable alternative to conventional methods, particularly through approaches such as mass stabilization and deep soil mixing. While the incorporation of Portland cement can significantly improve soil mechanical behavior, even in minimal quantities, its production is highly carbon-intensive, contributing approximately 5% to 7% of global CO₂ emissions. Furthermore, cement-stabilized soils tend to exhibit brittle behavior, which can compromise their performance. To address these limitations, this study explores an environmentally friendly alternative binder with a lower carbon footprint: a geopolymer based on granulated blast furnace slag (GBFS), activated with sodium hydroxide, and reinforced with sisal fibers. Laboratory specimens, with and without fibers, were cured for 28 days under controlled conditions of humidity and temperature. Unconfined compressive strength (UCS) tests were conducted to assess the mechanical behavior of the composite material. The results indicate that the geopolymer binder (GBFS + activator) achieves higher unconfined compressive strength than Portland cement. While the inclusion of sisal fibers reduced unconfined compressive strength in cement-stabilized soil, it had a positive effect when incorporated into geopolymer-stabilized soil. Regarding brittleness, the addition of fibers effectively enhanced the ductility of Portland cement-stabilized soil, whereas no significant improvement was observed in geopolymer-based samples. These results highlight the potential of fiber-reinforced geopolymers as a sustainable alternative for soil stabilization.