Reverse electrodialysis (RED) converts salinity gradient energy (SGE) into electrical energy. Seawater and river water are the most promising feed solutions for RED, but municipal wastewater instead of river water could be acceptable if the wastewater treatment plant is close to the sea because river water is often used for human demands, i.e., for drinking water and agricultural irrigation. In addition, the environmental impact at the mixing point (the brackish water region) must be considered. Wastewater pretreatment is necessary to prevent performance reduction in the subsequent RED process. In this study, the effect of pretreatment using polyaluminum chloride (PAC) as a coagulant on the RED performance was investigated using model and real municipal wastewater and seawater samples. The results reveal that the presence of PAC residue after coagulation negatively affected the subsequent RED process, especially because of the increased cation exchange membrane resistance, which resulted in a 40% reduction in power generation. However, the optimized PAC dosage (and lower amount of PAC residue) resulted in a 20% improvement in RED power generation because 50% of the wastewater organic content had been removed. Thus, coagulation pretreatment with the optimized PAC dose is useful before RED power generation when using municipal wastewater.