In the 1750s, and again in the 1780s, Portugal and Spain commissioned mapping expeditions to draw a border between Brazil and Spanish South America. The two Iberian courts hoped to resolve long-standing disputes over territorial possession through the latest cartographic technologies, yet their proposed division ran through lands controlled by autonomous indigenous communities. This dissertation explores the relationship between the subsequent mapping expeditions and interethnic relations in the Río de la Plata region – Uruguay, northeastern Argentina, and the far south of Brazil. Recent work on the history of cartography shows that maps were powerful tools of imperial governance, while scholarship on interethnic borderlands in the Americas suggests that imperial borderlines had little to no impact on native peoples until the nineteenth century. I contribute to this discussion by arguing that mapped lines were significant in certain eighteenth-century borderlands, but mainly because native peoples appropriated them for their own purposes. I draw upon manuscript materials from twenty-six archives in seven countries, and use Geographic Information Systems (GIS), to demonstrate the centrality of independent indigenous communities to the entire bordermaking process. At the time of the mapping expeditions, native peoples known as Charrúas and Minuanes were the principal arbiters of the Río de la Plata’s rural interior, restricting Iberian and Jesuit-Guaraní settlers to its perimeter. Given their limited territorial reach, Portuguese and Spanish diplomats turned to mapmaking as a means to claim native lands without having to claim native peoples as vassals. The mapping expeditions transformed imperial interethnic policies and engendered responses from Charrúas and Minuanes, who exploited Iberian bordermaking to expand kinship ties, establish commercial networks, and gain refuge in times of duress. These shifting territorial dynamics enabled some communities and caciques to expand their networks of power, while exposing others to capture and dislocation. Those who had prospered through the development of an operative borderline nonetheless found themselves debilitated when it began to dissolve in the early nineteenth century.