Organic soil is a problematic soil that needs to be treated before construction because of the low shear strength and high compressibility. Using by-product materials, such as fly ash (FA), to improve soils is a cost-effective and sustainable procedure. Because treatment with FA may lead to reduce shear strength, a FA-based geopolymer was used with a cohesive organic soil to substitute the reduction in strength. A series of unconfined compressive strength tests (UCS) were conducted on compacted specimens treated with FA and geopolymer. The geopolymer was produced by adding sodium hydroxide to activate the FA. Different levels of FA content, curing period, and temperature were applied to the specimens. The results indicate that for the FA treated specimens, the UCS decreased as the FA increased. For the geopolymer-treated specimens, as FA percentage in the geopolymer increased, the UCS increased and the axial strain at failure decreased. The optimum content of FA, in the geopolymer, was 20%, and the highest UCS was achieved at a curing period of 28 days at a temperature level of 65°C. Based on the obtained results, FA-based geopolymer can effectively be used to improve the strength of organic soils.