This article proposes a new conceptual tool for analysing how Southern actors engage in procedural and normative interventions within fragmented global climate governance. Termed Southern institutional agency (SIA), it captures the capacity of Global South coalitions to shape institutional outcomes despite structural constraints and entrenched normative hierarchies. Drawing on the case of the India–Brazil–South Africa (IBSA) Dialogue Forum, the study argues that Southern agency manifests not only through rhetorical positioning or bloc politics but also via coalitional diplomacy, procedural rule-shaping and norm subsidiarity. Through a structured analysis of IBSA’s engagements with the Green Climate Fund and the IBSA Fund for the Alleviation of Poverty and Hunger, the article illustrates how institutional design preferences and justice-orientated norms are gradually embedded into multilateral practices. The findings suggest that climate governance fragmentation, often portrayed as a liability, can open up spaces for procedural and normative interventions by Southern actors. The article offers both a conceptual framework and an empirical roadmap for rethinking agency in a post-hegemonic, contested multilateral landscape. In doing so, it contributes to climate diplomacy and South–South cooperation literatures by foregrounding institutional imagination and normative orientation as vital components of Global South influence.