Natural seed gum extracted from Cassia obtusifolia seeds was assessed to determine its potential as a coagulant aid together with alum to treat high-strength agro-industrial wastewater, namely palm oil mill effluent. If the untreated palm oil mill effluent was discharged into watercourses, it could cause severe environment pollutions due to its high total suspended solids and chemical oxygen demand. Thus, this study investigated the influence of alum dosage, C. obtusifolia seed gum dosage and settling time on both total suspended solids and chemical oxygen demand removals from the palm oil mill effluent using response surface methodology. The combined treatment between alum plus C. obtusifolia seed gum yielded total suspended solids and chemical oxygen demand removals up to 81.58 and 48.22 , respectively using the following optimum conditions: alum dosage of 1.15 g/L, C. obtusifolia seed gum dosage of 2.47 g/L and settling time of 35.16 min. Additionally, the combined treatment was observed to be an adequate alternative to treat palm oil mill effluent at its natural pH using relatively smaller alum dosage and shorter settling time in comparison with the treatment using C. obtusifolia seed gum alone. The presence of complexes in flocs and their requirement for higher degradation temperature (>220. ?C) were confirmed using fourier transform infrared spectroscopy and thermal gravimetric analysis, respectively. Scanning electron microscopy microphotograph revealed that the flocs after treatment using alum plus C. obtusifolia seed gum exhibited the presence of fibrous networks with rough surface and porosity.