Removal of reactive dyes from textile industry wastewaters is a great economic and environmental challenge. In this study, systematic adsorption tests were carried out using natural and modified bentonites to remove a textile dye, Procion Navy HEXL. The results indicated an optimum at 0.1% solids concentration and 4 h of mixing time. The modified bentonite (hexadecyltrimethylammonium bromide, HTAB) had a 12-fold increase in the adsorption capacity compared to the natural bentonite. The zeta potential experiments revealed that the negative surface charge of natural bentonite at all pH values became positive upon modification with HTAB. These results clearly indicated that the negatively charged dye can adsorb onto bentonite surface via electrostatic attraction. In addition, adsorption isotherm models (Langmuir, Freundlich and Dubinin–Radushkevich) were used to analyze the adsorption in the dye-bentonite system. It is concluded that bentonite after surface modification can be conveniently used in the abatement of reactive azo dyes in textile wastewaters.