No multilateral environmental agreement (MEA) has so far been concluded with a view to addressing the problem of ocean acidification. The United Nations Framework Convention on Climate Change (UNFCCC) is considered by many as being capable of addressing ocean acidification as it regulates carbon dioxide emissions – the root cause of the problem. In this article it is argued that, on the contrary, the UNFCCC does not provide an adequate legal framework for the problem because ocean acidification is not an effect of ‘climate change’, meaning that it is outside the UNFCCC's jurisdiction. The article provides a critical examination of whether ocean acidification is likely to be addressed through the self-organization of existing MEAs or whether a new MEA is necessary. Specifically, it considers the extent to which the provisions of relevant MEAs are applicable to ocean acidification and how their decision-making bodies have responded to the problem. This article observes inherent weaknesses in the emerging polycentric order and reaches the conclusion that a new MEA on ocean acidification is necessary to fill the regulatory gap. The article concludes by outlining two hypothetical candidates as a way of discussing key considerations informing the choice of an appropriate form and forum for an MEA on ocean acidification.