In this study, removal of colour from wastewaters prepared synthetically using Bomaplex Blue CR-L was investigated using raw and activated pumpkin seed shells by adsorption in a batch system. The effects of stirring speed, adsorbent concentration, dye concentration, temperature and pH on colour removal were investigated, and adsorption capacities of raw and activated pumpkin seed shells were determined. In addition, adsorption kinetics, isotherm coefficients, activation energies and thermodynamic parameters were calculated. The optimal adsorption conditions were determined as pH = 2, stirring speed of 200 rpm, adsorbent concentration of 10 g L−1 and 30 °C. Under the optimal conditions, the maximum removal efficiency of Bomaplex Blue CR-L using raw pumpkin seed shells was 73.01 %. This value rose to 79.71 % after activation processing was applied. Experimental adsorption data show that the adsorption is more suited to the Langmuir adsorption model and works in accordance with the pseudo-first-order kinetics model. As a result of thermodynamic research, ΔH° and ΔS° were 31.515 kJ mol−1 and 109.952 kJ mol−1 K−1 for the raw adsorbent, respectively. For the activated adsorbent, ΔH° and ΔS° were 43.118 kJ mol−1 and 152.237 kJ mol−1 K−1, respectively. The activation energy of adsorption was calculated as 10.918 kJ mol−1 for the raw adsorbent and 9.882 kJ mol−1 for the activated adsorbent.