Pulp and paper industry generates 30–180 m3 of wastewater per ton of manufactured pulp and 20–70 m3 of wastewater per ton of manufactured paper and paperboard. Coagulation process is widely applied as a pre-treatment or primary treatment to remove suspended solids from industrial effluent including pulp and paper mill effluent (PPME). Nevertheless, the excessive use of inorganic coagulants, such as alum, poses deleterious environmental impacts and risks to living organisms include low biodegradability, increase of metal content in discharged effluent and generation of toxic sludge. In view of this, the present study investigated the potential use of natural Cassia obtusifolia seed gum in treatment of raw and undiluted PPME through coagulation process. Recommended conditions (initial pH 5, 0.75 g/L dosage, 10 rpm and 10 min slow-mixing, and 1 min settling time) allowed C. obtusifolia gum removed high total suspended solids and chemical oxygen demand up to 86.9 and 36.2%, respectively. Findings from the present study showed that the coagulation efficiency using C. obtusifolia gum was comparable to alum. Also, characterizations showed that C. obtusifolia gum, alum and their flocs possessed distinctive features. The difference in peak occurrence from Fourier-transform infrared spectroscopy analysis indicated that the mechanism of floc formation using C. obtusifolia gum and alum differed. Besides, dissimilar thermal decomposition stages were observed for C. obtusifolia gum and alum through thermogravimetric analysis. Scanning electron microscope images showed that flocs formed using C. obtusifolia gum was highly fibrous-like and aggregate, whereas irregularly-shaped and aggregate for alum. In conclusion, C. obtusifolia gum could be served as a promising alternative to alum as a natural coagulant in treatment of PPME.