The growing global concern with environmental issues has raised the interest in the research into natural biopolymers as a coagulant aid in order to reduce the use of inorganic coagulants. This paper investigated the feasibility of sesbania seed gum (SSG) as a plant-based coagulant aid and ferric chloride as a coagulant in drinking water treatment. Acid extraction method marked the highest and most promising extraction yield at 20.8, as compared to other extraction methods. Further, the SSG extracted carried a weak negative charge of -3.02 mV, which is classified as a near neutral coagulant aid. Hydroxyl and carboxyl functional groups, which aid in coagulation-flocculation, were found in the SSG. These physiochemical analyses results evinced good characteristics of SSG as a coagulant aid. On the other hand, response surface methodology (RSM) with three-factor Box-Behnken design (BBD) was employed to evaluate and optimize the reaction condition of the coagulation-flocculation process in drinking water treatment. A quadratic polynomial model was fitted to the data with a high value of R2 (0.9901). Model validation experiments revealed the good correspondence between actual and predicted values. In drinking water treatment, a promising 98.3 turbidity reduction was achieved with 10.2 mg/L of FeCl3 and 4.52 mg/L of SSG. Therefore, SSG exhibited potential as a coagulant aid in drinking water treatment © 2020 by the authors.