In the past decade, research has increasingly highlighted the importance of political parties in protest arenas, yet has focused mainly on their visible roles in mobilisation. Analysing Portugal’s anti-austerity protests (2010–2014), this paper proposes that we should also analyse how parties engage actively behind the scenes. Through a protest event analysis and interviews, the article proposes that parties are not mere supporters or sponsors, but are also important active players in protest arenas. Leveraging their infrastructure and resources, they shape protest arenas via strategic backstage interactions; that is, they not only lend support but also are behind the scenes making and remaking protest arenas while keeping a low profile. These findings have broader implications for understanding the interplay between political parties and social movements. By highlighting hidden mechanisms of party influence, backstage partyisation challenges existing models that focus only on visible aspects of mobilisation and contributes to ongoing debates about the role of institutional players in social movements arenas. As such, this research emphasises the need to consider both public and backstage interactions to fully understand party-movement relationships.