In this study, the index and mechanical properties of perlite-geopolymer soils were examined through Atterberg limits tests, compaction tests, and unconfined compression tests. Perlite was added to the specimens at rates ranging from 0 to 50% based on the dry weight of the soil. The liquid limit and plastic limit of the specimens decreased as the perlite content increased. Increasing the perlite content of the soil led to a decrease in the optimum water content and the maximum dry unit weight of the specimens. The molarity effect of sodium hydroxide on the geopolymerization of perlite-added soil was also investigated. The presence of the liquid activator (sodium hydroxide) increased the unconfined compressive strength of the specimens. Among the specimens, those composed of 60% clay and 40% perlite, treated with a 4 M NaOH solution at 40 °C for 28 days, showed the best performance in terms of strength gain. Consequently, perlite-geopolymer soils were successful in increasing the strength of the soil. The treated soil transformed into a more rigid and stronger structure, greatly improving the geomechanical properties of the soil.