Hydroelectric power is obtained by harnessing the potential energy of a mass of water flowing along ariver bed to convert it first into mechanical and then into electrical energy. The system is therefore bothclean and autonomous and has a high potential for development.A small hydropower plant has an installed power capacity of below 10 MW and is considered run-of-river when it diverts part of a river flow to generate its electricity.The use of multicriteria techniques in environmental decision making, including selecting betweenvarious alternatives, is important when it involves complex decisions in a number of disciplinary fields.The most widely used of these techniques is the analytic hierarchy process (AHP).This work proposes an application of the AHP method to select the best alternative for the installation ofa small run-of-river hydropower plant. The systemization of the process, together with its mathematicalrigor, means the best project will be selected from the available technical, economic and environmentalalternatives.The main criteria identified in this study for the selection of RoR plant are: protected fauna, fish population, water quality, landscape quality, flow regime and vegetation. Finally, the best alternative was the project of the company Energia of Galicia.