Due to the toxicity of methylated and chlorinated phenols, it is necessary to remove them from aqueous solutions. Therefore, a highly efficient adsorbent was developed from the dead leaves of Halodule uninervis seagrass (SG) through a chemical and thermal activation process. The prepared activated carbon (SGAC) was characterized and investigated for removal of 2,4-Dimethylphenol (DMP) and 2,4-Dichlorophenol (DCP) from aqueous solutions. The batch adsorption mode was used and the effects of adsorption conditions were studied. SGAC adsorbent showed high adsorption capacities which were 364 mg DMP/g and 333 mg DCP/g. The adsorption experimental results fitted Freundlich and Langmuir models and followed pseudo-second-order kinetics. The adsorption of both adsorbates was spontaneous. The prepared adsorbent was able to remove 90% of DMP and 97% of DCP from the synthetic wastewater. This study indicated that SGAC adsorbent is efficient and reusable in the synthetic wastewater for at least four times.