We studied the hydrodynamics of Montevideo Bay using a numerical approach. The RMA-10 model was implemented and calibrated in a vertical averaged mode for the Río de la Plata estuary to simulate the velocity field in Montevideo Bay. By conducting simultaneous analyses of the flow conditions at several control points situated in the bay, we identified the most frequent circulation patterns. We identified two circulation patterns that explain the water exchange between Montevideo Bay and the Río de la Plata. These patterns represented the flow conditions during 80% of the simulated period. In a more detailed analysis, six circulation patterns were identified in the interior of Montevideo Bay. These patterns occurred during 62% of the simulated period. We also studied the influence of the different forcings acting on water circulation in the bay: river flow from the tributaries, astronomical tidal waves and meteorological tides entering from the ocean, and local winds. The numerical model that we used helped us to understand the complex interaction between these factors and processes in the Río de la Plata and Montevideo Bay. Finally, we calculated the residual circulation fields for different scenarios with regard to forcing. The resulting characterization is very relevant to the environmental management of the bay and to engineering projects in Montevideo’s harbor.