A high-performance porous biochar adsorbent prepared by facile thermal pyrolysis of seaweed (Gelidiella acerosa) is reported. The textural characteristics of the prepared seaweed biochar (SWBC) and the performance in the adsorption of methylene blue (MB) dye were evaluated. The batch experiment for the adsorption of MB was conducted under different parameters, such as temperature, pH, and initial concentration of MB in the range of 25-400 mg/L. The developed SWBC exhibited a relatively high surface area, average pore size, and pore volume of 926.39 m(2)/g, 2.45 nm, and 0.57 cm(3)/g, respectively. The high surface area and pristine mineral constituents of the biochar promoted a high adsorption capacity of 512.67 mg/g of MB at 30 degrees C. The adsorption isotherm and kinetics data best fitted the Langmuir and pseudo-second-order equations. The results indicate that SWBC is efficient for MB adsorption and could be a potential adsorbent for wastewater treatment.