The increase in intermittent sources of electricity in national grids is increasing the need for energy storagesolutions. Brazil’s energy grid has a hydrothermal conformation that the provision of power to meet the demand iseasily met with its numerous hydropower generators if there is water available to turn their turbines. With the intent ofstoring water and energy with a small flooded area for a whole cascade of hydropower plants, this paper proposes theconstruction of a new seasonal pumped storage plant (SPHS), named São Tomé SPHS, at the head of the Antas river inthe Rio Grande do Sul state. Results show that the proposed SPHS plant would increase the hydropower in the dams incascade in around 260 MW. Apart from this, the SPHS plant could also be used to store energy generated from solarand wind sources and generate electricity during peak hours.