The ecological flow is the minimum flow of a river necessary to maintain its aquatic fauna. In the present study, this flow was estimated for the Preto River, which belongs to the Piabanha River Basin, located in the state of Rio de Janeiro, Brazil. Because the Preto River’s entire length is within the state, its management is under the responsibility of the Rio de Janeiro State Environmental Institute (INEA). The INEA adopts the value of 50% of Q7,10 as the minimum ecological flow for rivers in the state. Hydrological data were obtained from a stage gauge located on the river, allowing estimation of the ecological flow. The results were satisfactory, because the two ecological flow values calculated were lower than the lowest flow observed during the historic discharge series of the river.