Microporous fish waste-based activated carbon material (MFC) was prepared, with a large surface area of 2,193.52 m²/g, a pore size of 2.67 nm and micropore and total pore volumes of 0.9168 cm³/g and 0.9975 cm³/g, respectively. Adsorption efficiency of MFC was investigated by removal of methylene blue dye from wastewater. The Langmuir model and pseudo-second-order kinetics adequately described the adsorption process. MFC exhibited a high adsorption capacity of 476.19 mg/g at 30 °C, and reached equilibrium within 1 h. MFC could be an efficient and low-cost adsorbent for cationic dye removal during wastewater treatment.