In order to promote good water governance practices, harmonize multiple objectives, and reflect the linkages between environmental, socioeconomic and politico-administrative aspects, it is imperative to define appropriate regions for water resources planning and management. The Water Resources Planning and Management Decision Support System (WARPLAM DSS) is presented here as a tool to support the definition of suitable limits for water resources regions. Although river basins are generally considered the most suitable regions to achieve Integrated Water Resources Management (IWRM) goals, WARPLAM DSS facilitates the analysis of politico-administrative, historic-cultural, socioeconomic, and physical-environmental aspects. This paper summarizes an initiative to define integrated water resources regions for the establishment of River Basin Committees (RBCs) in Peru, based on a comprehensive analysis of the Peruvian territory. The study was developed through a partnership between ANA-Peru and ANA-Brazil, with the cooperation of the Inter-American Development Bank (IDB). The results simulated in this attempt anticipate a more harmonized division of Peru's territory into water resources regions. Such an approach aims to establish more robust RBCs and to increase the chances of attaining IWRM goals.