A peculiar treaty management organization operates under the UNFCCC and the Paris Agreement that does not fit established categories of international law and political science. Unlike traditional international organizations (IOs), it lacks the formal status of an IO, comprises only a limited secretariat with predominantly servicing functions, and is not even denominated as an organization. We argue that it has nevertheless become an international actor, mainly due to activities of its Conference of the Parties and several specialized organizational schemes. Theoretically, we develop an analytical framework that shows how even heavily member-dominated IOs can become international actors and what this means for global environmental governance. IOs gain the capability to influence international politics by their own action if authorized to make decisions with external effects. They gain autonomy if organizational rules and procedures shape organizational decision-making and create specific organizational rationales. Empirically, we demonstrate that the organizational component of the UNFCCC and the Paris Agreement has acquired both considerable action capability and autonomy. It is authorized to flesh out the skeletal provisions of the constituent treaties through far-reaching COP decisions and to decide continuously in several specialized organizational schemes, especially on climate funding, cooperation mechanisms, and compliance management. Organizational decisions are heavily influenced by autonomy-creating organizational factors, such as path dependence, fundamental organizational norms and dense sets of decision criteria. We conclude that this organization, and the organizational components of other multilateral environmental agreements, point at important organizational effects, which merit further attention.