Negotiators preparing for Rio+20 are missing an important opportunity. Private sustainability governance (PSG) is thriving: organizations created by business and civil society groups, as well as public–private partnerships, adopt and apply significant regulatory standards and undertake valuable operational activities, including pilot projects and financing. However, even though reforming the institutional framework for sustainable development is a central part of the Rio+20 agenda, negotiators are focusing almost exclusively on inter-governmental organizations such as the UN Environment Program (UNEP), the Commission for Sustainable Development and the Economic and Social Council. This public–private engagement gap isolates international governance from the energy and innovation of PSG, and impedes efforts to coordinate the bifurcated and decentralized system of sustainability governance. This article argues that states, and especially international organizations, should actively support PSG as part of the institutional framework for sustainable development, while steering private and public–private schemes towards good organizational practices and the pursuit of public goals. Engagement with PSG would help international institutions pursue their sustainability missions more effectively, promote the emergence of effective and legitimate private schemes, manage fragmentation, promote experimentation and learning, and enhance citizen participation. The article outlines two fruitful modes of engagement pioneered by UNEP: regulatory cooperation, in which international authorities engage directly with business firms, industry groups and other ‘targets’, influencing them to adopt more sustainable behaviors; and orchestration, in which authorities engage with intermediary organizations, such as multi-stakeholder private governance schemes, catalyzing, supporting and steering them as they seek to influence the ultimate targets of policy.