This paper seeks to illuminate the multiple ways in which South–South collaboration may reorganize knowledge production and learning processes across scales and beyond the unilateral transfer of expertise. Drawing on empirical evidence from a knowledge exchange partnership between water and sanitation operators in Salta, Argentina, and Brasília, Brazil, we provide a grounded, contextual account of the partnership to examine what was learned, under what circumstances, and with what potential effects. We contend that common claims by proponents of South–South cooperation around the centrality of shared geopolitical history are not enough to understand South–South cooperation at the project level. At this scale, we find that other forms of proximity, including organizational, linguistic, technological, and cultural, also matter in shaping the constitution of collaborative partnerships and the forms of learning that occur through them. In the case that we examine, partners’ multiple shared proximities resulted in a subversion of traditional mentor–mentee relations and emergence of a process of mutual learning. Further, we suggest that flows of knowledge in the partnership can be characterized across a learning spectrum, from technical and processual learning to experiential understanding and self-reflection, each with different consequences for institutional and material change at different scales. Crucially, such forms of learning bolstered participants’ bargaining power for implementing improvements at home and fostered advances in operators’ tactical thinking.