This article presents an analysis of the multilateral system, arguing that multilateralism is going through a profound set of changes as a result of: (1) the emergence of new multilateral actors; (2) the development of new multilateral playing fields; and (3) the rise of new concepts of multilateralism. This has consequences for world politics: the world is moving from unipolarity towards a networked form of multipolarity. This article proposes to grasp these changes through the ‘Web 2.0’ metaphor, as the existing multilateral system is contrasted with the emerging ‘Mode 2.0’ of which the main characteristics are: (1) the diversification of multilateral organisations; (2) the growing importance of nonstate actors such as substate regions and supranational regional organisations; (3) the increased interlinkages between policy domains; and (4) the growing space for citizen involvement. The main upshot is that the multilateral system is moving from a closed to an open system. Both states and international organisations will have to adapt to this new reality.