Contamination of the world's water resources is a growing issue and its remediation requires the development of highly efficient, environmentally friendly and economical processes. Water contaminated with surfactants can cause detrimental health effects in humans and aquatic animals. Removing surfactants using adsorption is effective, simple and economical. This paper describes the development of a fly ash based geopolymer (FAGP)adsorbent for adsorbing the anionic surfactant sodium dodecyl benzene sulfonate (SDBS). The adsorption parameters of the geopolymers were optimized using batch adsorption. The adsorption kinetics, isotherms and thermodynamics were also determined. The FAGP had an amorphous morphology and a surface area of 31.873 m2/g. The optimum parameters for adsorbing the SDBS using FAGP were pH 2, contact time 180 min, adsorbent dosage 1 g/L for an initial SDBS concentrations of 880 mg/L. The maximum adsorption capacity of 714.3 mg/g was obtained. The adsorption followed pseudo second order kinetics and Langmuir isotherm models suggesting that the adsorption process was chemisorption with monolayer adsorbate coverage. SDBS was adsorbed onto FAGP by electrostatic interactions between the positively charged FAGP and negatively charged SDBS. The activation energy of adsorption was 4.052 kJ/mol and the Gibbs free energy was negative, suggesting the adsorption process was physisorption, endothermic, spontaneous and more favorable at a temperature of 65 °C. The adsorption of SDBS onto FAGP occurs through both physisorption and chemisorption. FAGP was proven as a low-cost adsorbent to remove SDBS and could be potentially used for the adsorption of other water contaminating surfactants. © 2019 Elsevier Ltd