This paper theorizes about the convergence of international organizations in global health governance, a field of international cooperation that is commonly portrayed as particularly hit by institutional fragmentation. Unlike existing theories on interorganizationalism that have mainly looked to intra- and extraorganizational factors in order to explain why international organizations cooperate with each other in the first place, the paper is interested in the link between causes and systemic effects of interorganizational convergence. The paper begins by defining interorganizational convergence. It then proceeds to discuss why conventional theories on interorganizational- ism fail to explain the aggregate effects of convergence between IOs in global (health) governance which tend to worsen rather than cushion fragmentation — so-called "hypercollective action" (Severino & Ray 2010). In order to remedy this explanatory blind-spot the paper formulates an alternative sociological institutionalist theory on interorganizational convergence that makes two core theoretical propositions: first that emerging norms of metagovernance are a powerful driver behind interorganizational convergence in global health governance, and secondly that IOs are engaged in a fierce meaning-struggle over these norms which results in hypercollective action. In its empirical part, the paper’s core theoretical propositions are corroborated by analyzing discourses and practices of interorganizational convergence in global health. The empirical analysis allows drawing two far-reaching conclusions. On the one hand, interorganizational harmonization has emerged as a largely undisputed norm in global health which has been translated into ever more institutionalized forms of interorganizational cooperation. On the other, discourses and practices of interorganizational harmonization exhibit conflicts over the ordering principles according to which the policies and actions of international organizations with overlapping mandates and missions should be harmonized. In combination, these two empirical findings explain why interorganizational convergence has so far failed to strengthen the global health architecture.