The present study was conducted to optimize the various experimental conditions, such as biomass loading, initial C.I. Acid Blue 25 (AB25) dye concentration, and initial solution pH for biosorption of dye on raw and HCl-treated brown alga, Padina australis and red alga, Jania adhaerens. Biosorption process was optimized in a batch system under Box-Behnken design. Second-order polynomial equation was successfully used to describe the effects of studied variables on response. The quadratic models exhibited higher R-2 values, significant p-values, and insignificant lack-of-fit p-values showed high adequacy for predicting the response. Chemically modified red alga exhibited better AB25 dye biosorption capacity as compared to modified brown alga. Maximum dye removal efficiencies of 77.34, 71.28, 50.56, and 85.19% for P. australis, HCl-treated P. australis, J. adhaerens, and HCl-treated J. adhaerens, respectively, were obtained at optimal conditions. The surface modification on tested algal biomass was found to be strongly dependent on their cell wall constituents.