This study focused on determining the effect of powdered activated carbon on treatment performance and membrane fouling for pharmaceutical wastewater treatment by using an anaerobic membrane bioreactor system. The anaerobic membrane bioreactor system was operated without powdered activated carbon addition for 131 days and with powdered activated carbon addition for 169 days. The anaerobic membrane bioreactor performance was evaluated considering the chemical oxygen demand concentration, flux value, biomass concentration, extracellular polymeric substances and soluble microbial product concentrations. In the period operated without the powdered activated carbon addition, the chemical oxygen demand removal efficiencies were in the range of 67–70%, whereas the removal efficiency of chemical oxygen demand increased up to 96% after the powdered activated carbon addition. At the beginning, the system was operated with the flux of 3.2 L/m2 h and the flux decreased to 0.4 L/m2 h on the 131st day. Although the chemical oxygen demand removal performance increased, the addition of powdered activated carbon was not effective in improving the flux. In the study, scanning electron microscope, energy-dispersive X-ray spectroscopy, confocal laser scanning microscopy, optical profilometer measurements and fouling modeling studies were conducted to identify the mechanism of fouling on the membrane. According to these analyzed results, the predominant fouling mechanism was cake layer formation for the fouled membrane used in the period of operation without the powdered activated carbon addition. It was observed that the thickness of the cake layer accumulated on the membrane surface slightly decreased and the cake layer became smoother with powdered activated carbon addition. When the membrane fouling model was evaluated, it was clearly revealed that the cake layer adsorption was dominant before the powdered activated carbon addition. Although the dominant fouling mechanism shifted to the intermediate adsorption with the powdered activated carbon addition, the cake filtration and intermediate adsorption mechanisms were predominant simultaneously.