This paper presents findings from an experimental study in which a novel, non-energy intensive, environmentally-friendly stabilizer known as geopolymer was used for stabilization of a highly plastic clay. Two forms of the stabilizer were synthesized, one using metakaolin (MK) and the other using fly ash (FA) as the alumino-silicate precursor. The paper describes the process of geopolymer synthesis as well as quality control tests conducted during geopolymer synthesis. Synthesized geopolymers were analyzed using scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDX). The paper also presents results obtained from a comprehensive laboratory test program that examined the effectiveness of the stabilizer in improving strength and controlling moisture induced swelling of a highly plastic clay soil. The geopolymer content of the soil ranged from 3% to 15% by weight ratio. SEM-EDX and XRD were performed to investigate the treated specimen for uniformity and stabilization mechanism. The data obtained from unconfined compressive strength tests conducted after seven days of curing showed three- to four-fold improvement in strength for geopolymer stabilized soils with MK geopolymer consistently providing better performance than FA geopolymer. The results from one-dimensional swell tests indicated significant reduction of swell behavior in FA geopolymer treated specimens while no improvement in swell behavior was observed in MK treated soil. This study demonstrates that, while geopolymers can overcome many limitations that exist in traditional stabilizers, the strength and swell performance of geopolymer stabilized clay soil can vary significantly depending on the source of alumino-silicate used in the production of the geopolymer.