This study is the first comprehensive life cycle assessment (LCA) of reverse electrodialysis (RED), a technology that converts salinity gradient energy into electricity. Our goal is to identify RED system components of environmental concern and provide insights on potential environmental impacts. We conduct an attributional LCA of two RED scenarios: large-scale energy generation from natural bodies of water and smaller-scale energy generation from industrial processes. A functional unit of 1 MWh of net electricity production enables comparison to existing renewable energy technologies, including wind and solar photovoltaics. Under theoretical, favorable conditions, environmental impacts from RED are found to be comparable to, and often lower than, established renewable energy technologies. Processes associated with membrane manufacture are primary contributors to six of the nine evaluated impact categories. Under baseline assumptions, impacts are an average of 50% higher for the Natural Water scenario compared to the Concentrated Brine scenario because of the increased power density achieved with concentrated brines. This early-stage LCA demonstrates that the expected environmental impacts of RED are comparable to existing renewable technologies and a large improvement over fossil-based generation. However, eutrophication, ecotoxicity, and carcinogenic impacts are larger for RED than other technologies under some assumptions.