Expansive soils pose significant challenges in civil engineering projects due to their propensity to undergo volumetric changes with variations in moisture content, leading to detrimental effects on structures, such as foundations, roads, and pavements. This study investigates the potential of Prosopis juliflora ash (PJA) as a stabilizing agent for improving the engineering properties of expansive soil. PJA, a by-product of biomass combustion, is readily available in regions where Prosopis juliflora is abundant. Laboratory experiments were conducted to evaluate the effects of PJA on the index properties and engineering properties of expansive soil. Various proportions ranging from 0 to 20% of PJA were added to the expansive soil with and without lime. Tests, such as Atterberg’s limits, compaction, swell characteristics, and unconfined compressive strength (UCS), were performed. The results were compared with those of untreated soil to assess the effectiveness of PJA in mitigating the expansive behaviour of the soil. The findings indicate that the addition of PJA led to improvements in the plasticity characteristics of the expansive soil. Significant increase in strength was observed in the UCS values with the addition of 20% PJA, leading to the stabilization of the expansive soil. Addition of 6% lime + 20% PJA further enhanced UCS values. Hence, it is concluded that the utilization of Prosopis juliflora ash serves as a sustainable and cost-effective solution for ameliorating expansive soils.