This paper analyzes the exercise of Brazilian hydrohegemonyin relation to the binational Itaipu hydroenergy plant on the transboundary Parana river, and the Paraguayan diplomatic strategies for the renegotiation of the Itaipu Treaty between 2008 and 2009. The Itaipu Treaty, signed in 1973, is the document that rules the relations between Brazil and Paraguay related to the share of the hydroelectric power produced by Itaipu. The rise of left-wing governments in both Brazil and Paraguayin the first decade of the twentieth-first Century, with a foreign policy aimed at fostering South American regional integration opened the room for Paraguay’s diplomacy to renegotiate the terms of this treaty. Despite the ideological affinities between both governments , the negotiation process was not free from conflicts. From the analysis of data collected in interviews and research at the digital collection of the Paraguayan newspaper ABC Color, it was possible to trace the genesis of the idea of “hydroelectric sovereignty” in Paraguay leading to the development of an “ideational” expression of power, and the exercise of material power on the part of Brazil during the negotiations. The article is divided in four parts. The first part presents the idea of hydroelectric sovereignty, identified with a realistic view of International Relations. The second shows the construction of the claims for greater sovereign participation by Paraguay through the analysis of publications of the ABC Color newspaper. The third part presents the first negotiation attempts, still under the Paraguayan government of Nicanor Duarte Frutos. Finally, it presents the changes negotiated between Brazil and Paraguay during the governments of President Fernando Lugo, Paraguay, and Luis Inácio Lula da Silva, Brazil. Keywords: Brazil/Paraguay; hydrohegemony; hydroelectric sovereignty; international cooperation; transboundary waters; water conflicts;Itaipu hydroenergy plant.