The coal bottom ash (CBA) sourced from thermal power plant was chemically treated using hydrochloric acid (HCl), namely CBA-HCl, and used as an adsorbent for palm oil mill effluent (POME) decolourisation. The treated CBA-HCl was characterised for surface property via Brunauer-Emmett-Teller (BET), surface chemistry via Fourier transform infrared (FTIR) and surface morphology via scanning electron microscopy (SEM). Its effectiveness for adsorption of POME colour was investigated via batch experiments as a function of pH, initial POME concentration, adsorbent dosage and contact time. The adsorption increased with increasing contact time until an equilibrium state achieved within 18 h to 24 h. The maximum POME colour removal, 81.15% was achieved at pH 6 utilising 14% (w/v) adsorbent dosage for 24 h. The colour adsorption isotherm and kinetics of the CBA-HCl was well-fitted with Freundlich and pseudo-second order models with correlation coefficient, R2 = 0.9991 and R2 = 0.9996, respectively. Based on the results, CBA can be a potential adsorbent in removing POME colour before being discharged into a watercourse.