The main purpose of this study is to modify the petroleum coke sample (PCS) for the first time by oxygen plasma treatment (OPT) as an alternative to the existing surface modification methods and to remove the malachite green (MG) colourant from the aqueous medium with the obtained adsorbent. The changes caused by OPT on the PCS surface were observed with photoelectron spectroscopy(XPS), infrared spectroscopy(FTIR), scanning electron microscopy(SEM) and surface area analysis. The atomic percentages of the oxygen for the raw and plasma treated sample were found at 4.021% and 9.363%, respectively. In the light of the obtained results, it was seen that new active groups containing oxygen on PCS increased by OPT. The functional groups such as -COOH, -C-O and -C = O with oxygen plasma on PCS were interpreted to the mechanism possible. The adsorption capacity of MG was 66.66 mg/g at 30 degrees C for the PCS treated with oxygen plasma. The Delta G, Delta H and Delta S values were 168.76 kJ/mol, 84,49 kJ/mol and -278,19 J/mol K.