Calcium alginate hydrogel was grafted with poly (styrene-co-maleic anhydride) synthetic polymer (PSMA) and used as an adsorbent for the effective removal of methyl violet 6B cationic dye from aqueous solutions. The characteristics of native and grafted alginate hydrogels were investigated using FTIR, Zetasizer and TGA/DSC. The carboxyl groups' content and the swelling properties were determined as well. Batch adsorption experiments were conducted as a function of initial dye concentration, adsorbent dosage, solution ionic strength, solution pH, time of contact and temperature. Results revealed that grafting of PSMA onto alginate improved the removal percentage of the dye up to 30%. The highest adsorption capacity of the dye was obtained at a temperature of 40 degrees C, a pH range of 5-11 and at lower solution ionic strengths. The kinetics of adsorption followed the pseudo-second-order model and the equilibrium data could be better described by the Langmuir isotherm. The maximum adsorption capacity was found to be 109.9mg/g suggesting the promising potential of our low-cost adsorbent for the removal of cationic dyes from aqueous solutions. A desorption study was carried out where the adsorbent showed high desorption characteristics and it could be reused at least for five consecutive cycles.