Over the last 3 decades, European cities have strengthened their cooperation at the European Union (EU) level to tackle common problems. The joint action of cities has been particularly concerned with the issue of sustainability, signaled by the establishment of specific municipal networks. Socioecological urban networks (SEUNs) have attracted growing academic attention. However, existing research presents two main limitations: firstly, it overlooks the urban context and, secondly, it is mainly qualitative, and the few quantitative studies do not provide an exhaustive account of the urban drivers underpinning SEUN membership. This article adopts an urban approach to isolate the urban-level economic, political, and institutional factors that impact on the involvement of second- and third-tier cities in European SEUNs. For this, logistic and ordinary least squares (OLS) regression models are used to identify the urban-level factors affecting SEUN membership. The findings show that interurban networking in Europe is an economic and political strategy adopted mostly by postindustrial cities to strengthen their profile.