Many industries produce 2,4-dimethylphenol (DMP) compound in the wastewater which is persistent, toxic, and carcinogenic. Therefore, an adsorbent was prepared by carbonizing a dried Sargassum boveanum macroalgae. The prepared biosorbent was investigated for adsorptive removal of DMP from aqueous solution. After carbonization, the biochar derived from S. boveanum macroalgae (BCM) removed almost 100% of DMP adsorbate. Effects of contact time, solution pH, adsorbate concentration, adsorbent mass, and temperature have been studied. It has been found that, within the experimental conditions, the maximum adsorption capacity is 17 mg/g, rate of adsorption follows pseudo-second order kinetics and the adsorption isotherm experimental data fit the Freundlich model. The thermodynamic parameters were calculated and it has been found that the adsorption of DMP on BCM is endothermic and thermodynamically favorable, and in addition the surface of BCM adsorbent shows affinity to the DMP molecules. The BCM adsorbent has the capability to remove around 65% of DMP from high saline seawater contaminated with DMP. Moreover, the prepared BCM adsorbent was reusable for at least four times in seawater for removal of DMP contaminant.