Coagulation is an important step in the physicochemical treatment of wastewater. Due to the disadvantage associated with the commonly used inorganic metal salts and other synthetic polymers, in recent years, research on the use of natural organic polymers as coagulant is gaining importance. In this study, the coagulation potential of alginate extracted from brown algae, Sargassum sp. for the removal of Congo red dye from aqueous solution has been identified. The yield of alginate extraction was found to be 40.8%. The extracted alginate was characterized by Fourier transform infrared spectroscopy and Scanning electron microscopy techniques. The effect of initial pH (4–6), alginate dose (10–60 mg/L), calcium dose (1–6 g/L), and initial dye concentration (50–250 mg/L) on dye removal have been investigated. It is inferred from the study that the maximum removal of dye (96%) was achieved with increasing alginate and calcium dose for increasing dye concentration, at pH 4. The obtained results were compared with the literature on Congo red dye removal using various adsorbents and coagulants. © 2015 Balaban Desalination Publications. All rights reserved.