Paradiplomacy, understood as the international engagement of subnational actors, has proven to be a relevant element in promoting regional integration within the framework of Mercosur. This study is based on the hypothesis that there is a two-way relationship between the strengthening of regional integration and the emergence of city networks, such as the Mercocities Network. It is argued that decentralized cooperation is not only a result of regionalism processes but also acts as a catalyst for them. The findings suggest that, despite structural limitations, the international action of South American cities contributes to dynamizing and territorializing integration within the region and the Mercosur bloc. In this context, the role of paradiplomacy is investigated through a case study of the Mercocities Network, with the aim of understanding how its initiatives contribute to strengthening local public policies. The analysis shows that city networks can serve as strategic instruments for regional integration, reinforcing the importance of decentralized cooperation in the Global South.