The phosphorus (P) adsorption characteristic of sesame straw biochar prepared with different activation agents and pyrolysis temperatures was evaluated. Between 0.109 and 0.300 mg L⁻¹ in the form of inorganic phosphate was released from raw sesame straw biochar in the first 1 h. The release of phosphate was significantly enhanced from 62.6 to 168.2 mg g⁻¹ as the pyrolysis temperature increased. Therefore, sesame straw biochar cannot be used as an adsorbent for P removal without change in the physicochemical characteristics. To increase the P adsorption of biochar in aqueous solution, various activation agents and pyrolysis temperatures were applied. The amount of P adsorbed from aqueous solution by biochar activated using different activation agents appeared in the order ZnCl₂ (9.675 mg g⁻¹) > MgO (8.669 mg g⁻¹) ⋙ 0.1N-HCl > 0.1N-H₂SO₄ > K₂SO₄ ≥ KOH ≥ 0.1N-H₃PO₄, showing ZnCl₂ to be the optimum activation agent. Higher P was adsorbed by the biochar activated using ZnCl₂ under different pyrolysis temperatures in the order 600 °C > 500 °C > 400 °C > 300 °C. Finally, the amount of adsorbed P by activated biochar at different ratios of biochar to ZnCl₂ appeared in the order 1:3 ≒ 1:1 > 3:1. As a result, the optimum ratio of biochar to ZnCl₂ and pyrolysis temperature were found to be 1:1 and 600 °C for P adsorption, respectively. The maximum P adsorption capacity by activated biochar using ZnCl₂ (15,460 mg kg⁻¹) was higher than that of typical biochar, as determined by the Langmuir adsorption isotherm. Therefore, the ZnCl₂ activation of sesame straw biochar was suitable for the preparation of activated biochar for P adsorption.