During the second half of the eighteenth century, Portugal and Spain commissioned two mapping expeditions to determine a border between Brazil and Spanish viceroyalties, agreeing for the first time to define territorial possession through collaborative cartographic efforts. These efforts were mediated by autonomous indigenous communities, who asserted their own land claims. This article explores this dynamic at the borderline's southernmost portion, an area corresponding to present-day Uruguay, northeastern Argentina, and southern Brazil. I argue that native peoples known as Charrúas and Minuanes appropriated imperial border-making efforts for their own purposes. As royal officials sought to materialize a border in lands that they did not effectively control, they solicited native agents' support. In response, Charrúas and Minuanes took up arms, crisscrossed the border to develop informal economies or elude imperial armies, or sought to incorporate new settlers into indigenous sociopolitical networks. These actions undermined imperial designs yet made the border a meaningful form of territorial organization.