Abstract The Brazilian and US labor movements are currently confronting common challenges related to changes in the overall structure and profile of the working class, the elimination of traditional sources of financing, and the weakening of allied political forces in the electoral arena. Considering these joint trajectories, the authors will examine recent efforts by US-based and Brazilian labor movement actors to create spaces for strategic collaborations, two-way learning, and mutual solidarity, to support common political goals and efforts towards union renovation. The article analyzes three contemporary cases of US-Brazil union solidarity networks, in order to better understand how transnational union activism influences national-level outcomes. The authors posit that the success of these forms of activism is dependent on objective limits created by external economic and political conditions, yet in some circumstances non-transactional solidarity is capable of producing unexpected positive outcomes for workers and their union organizations.