This data article presents a simple method for providing a biosorbent from Sargassum oligocystum harvested from the northern coast of Persian Gulf, Bushehr, Iran. The characterization data of Sargassum oligocystum biochar (SOB) were analyzed using various instrumental techniques (FTIR and XPS). The kinetics, isotherms, and thermodynamics data of Hg2+, Cd2+, and Cu2+ ions onto SOB were presented. The maximum biosorption capacity of SOB to uptake Hg2+, Cd2+, and Cu2+ ions from aqueous solution was obtained 60.25, 153.85, and 45.25 mg/g, respectively. The experimental data showed that biochar prepared from Sargassum oligocystum is an efficient and promising biosorbent for the treatment of heavy metals-bearing wastewaters.