Marine renewable energy development aims to harness the vast resources of the coastal environment to meet growing energy demands. Among the variety of coastal energy sources, salinity gradient energy (SGE) technology captures the energy released from the controlled mixing of waters of different salinities found naturally in estuarine systems or in other combinations of anthropogenic sources of brine and fresh waters. Although SGE technology is in the pilot stage of development, there is currently no comprehensive published assessment of its environmental impacts. This assessment uses research from proxy facilities and pilot SGE projects to predict impacts of an SGE facility through construction, operation and decommission phases. Construction and decommission of an SGE facility is expected to disrupt habitat and organisms via noise, land modification, and release of pollutants, similar to other coastal facilities such as desalination plants or water intake cooling structures. During operation, however, impacts unique to SGE result from intake of both high- and low-salinity waters, which in natural sources will involve impingement, entrainment, and chemical treatment to reduce biofouling. In addition, SGE effluent is expected to be of lower salinity and higher volume than effluent from a desalination plant, thereby posing unique risks to discharge sites. This review highlights areas of concern and lack of knowledge, which can guide environmental impact assessment and monitoring priorities of site managers.