Since the mid-2000s, the practice of South-South cooperation in health (SSC) has attracted growing attention among policymakers, health and foreign affairs ministries, global health agencies, and scholars from a range of fields. But the South-South label elucidates little about the actual content of the cooperation and conflates the “where” with the “who, what, how, and why”. While there have been some attempts to theorize global health diplomacy and South-South cooperation generally, these efforts do not sufficiently distinguish among the different kinds of practices and political values that fall under the South-South rubric, ranging from economic and geopolitical interests to social justice forms of solidarity. In the spirit of deepening theoretical, historical, and social justice analyses of SSC, this article: (1) critically revisits international relations theories that seek to explain SSC, exploring Marxian and other heterodox theories ignored in the mainstream literature; (2) traces the historical provenance of a variety of forms of SSC; and (3) introduces the concept of social justice-oriented South-South.