The present study emphasizes that biosynthesis of activated carbon (AC) from Borassus flabellifer, fruit husk waste (BFFHW) and improves the adsorption of reactive red-120 (RR-120) dyes. The biosorption of RR-120 was optimized using response surface methodology-Box–Behnken design (RSM–BBD) with three different parameters such as BFFHWAC concentration, RR-120 concentration, and time course of biosorption. The BFFHWAC pre and post-biosorption of RR-120 dye were characterized by field emission scanning electron microscope (FESEM), X-ray diffraction (XRD), and Fourier-transform infrared spectroscopy (FT-IR). The maximum biosorption (93.75%) was achieved with the BFFHWAC of 10 mg/L, pH 5.0, and a contact time of 120 min at room temperature. The experimental report fitted well to Freundlich adsorption isotherm (R2 = 0.96) and kinetics followed the pseudo-second-order model. The phytotoxic assay has confirmed the efficiency of the BFFHWAC in the removal of RR-120 from aqueous solutions using Vigna radiata seeds as the model. It suggests BFFHW based AC plays an important role on enhanced the removal of textile dyes from textile wastewater effluent.