This paper presents a two-dimensional hydrodynamic model of Patos Lagoon carried out within the framework of ECOSUD (Estuaries and Coastal Areas. Basis and Tools for a More Sustainable Development), a research project financed by the European Community. This lagoon is located in the southern Brazilian state of Rio Grande do Sul, between the cities of Rio Grande and Porto Alegre. It is approximately 250 km long, runs parallel to the coast and has an area of 10,360 km². The estuarine area is 60 km long. It receives major tributaries, which bring pollutants from farms and industry and also from waste from nearby cities like Pelotas and Rio Grande. Due to the importance of this resource and the problems presented for economic and recreational activities, numerical studies must be carried out to characterize the hydrodynamic behavior of various potentially productive areas such as Mangueira Bay. This hydrodynamic characterization complements water-quality studies in this area, and both act as inputs for aquaculture and water-resources planning. Different simulations were carried out with a two-dimensional model, RMA-2 (USACE), under a combination of several forcing agents during specific periods. These simulations were calibrated and validated with the available data.