The variety of available technologies and the low investment power in sanitation services, especially in regions with low and emerging economies, makes the selection of an optimal wastewater treatment system design an even more complex task for decision-makers. Thus, this study aimed to develop a multi-criteria analysis-based tool to support decision-making on the optimal wastewater treatment technology for the needs and priorities of each region, the Most Appropriate Treatment Technology Index (MATTI). The methodology to apply the MATTI comprises five steps: select suitable technologies; determine the effluent quality parameters; define the most relevant variables for design; normalize and define the weighting criteria; and calculate the level of compliance (0-1, on an increasing scale of suitability). To validate the tool, two different scenarios and seven variables for the Brazilian context were simulated. Different sets of technologies scored above 0.75, and were classified as highly recommended, according to the weight criteria attributed to each scenario. MATTI not only contributed to improving the decision-making process, but also with a more global vision of the parameters to be considered in the selection of technologies to meet the needs and priorities.