Lightweight cement materials are extensively used in the infrastructure construction. Geopolymer is a low-carbon and environmentally friendly cementitious material. This paper presents an investigation on the physical, hydraulic, and mechanical characteristics of lightweight geopolymer stabilized soil (LGSS) and a comparison with lightweight cement stabilized soil (LCSS). Measurements of volumetric absorption (VA) of water, hydraulic conductivity (k), and unconfined compressive strength (qu), scanning electron microscope (SEM) observation, mercury intrusion porosimetry (MIP) test, and thermogravimetric analysis (TGA) are conducted. The results show that LGSS has higher VA than LCSS. The k of LGSS is one order of magnitude higher than that of LCSS. The qu of LGSS is 2-3.5 times of that of LCSS. Microstructurally, the VA and k of LGSS are found to be positively correlated with the volume of large air pores (>10 μm). Higher qu of LGSS than LCSS is attributed to more hydration products that fill up the voids of soil. It is concluded that LGSS gives better engineering performances than LCSS in terms of water absorption, permeability, and strength characteristics.