The experiment with social participation in Porto Alegre—participatory budgeting (PB)—which came about under the progressive government of the Workers’ Party in 1989, is now one of the most widespread practices in the world. This device is present in about 2,800 municipalities, according to the most recent survey. In spite of this, in the literature produced on PB, the modalities of this global spread are imperfectly explained. How did PB move from a local experience to an object of massive diffusion and multiple appropriations? What was the role of international actors? To what extent has the initial project of PB in Porto Alegre been transformed through the process of international circulation? This article aims to answer these questions. At the crossroads of public policy analysis and international relations studies, the argument sustained in this article is, on the one hand, that the action of a set of individuals, called “ambassadors of participation,” was a necessary condition to introduce PB to the international agenda. On the other hand, international institutions such as the World Social Forum, the UN, the European Union, and the World Bank were fundamental in amplifying this spread so that it took on a global scale. This article is based on evidence collected through a transnational multisite fieldwork conducted in nine countries.