Since the 1990s, the progressive transformation of social-democratic parties into catch-all organizations, with a light ideological baggage and lack of social rootedness, has negatively influenced their relationship with the social movements. While losing their traditional institutional reference point, social movements are experiencing new forms of interaction with other party families – e.g. the Greens, the radical left and hybrid parties such as the Italian Five Star Movement. Accordingly, this study examines the ‘strategic interactions’ between the main ‘renewed’ (or ‘refounded’) radical left-wing parties (RLPs) and the left-wing social movements in Italy and Greece from 1999 to the present. The goal is to identify the processes by which the interactions between the two actors take shape, and the factors that contribute to success and failure in building them.  To this end, I take into account both the adaptive changes that the RLPs have enacted under the impulse of social movements and the reactions of social movements to those party transformations. First, I distinguish between three party dimensions – organization (structure and internal mechanisms), political culture (values and political issues), and strategies (alliances within the political system) – and verify whether social movements represented a stimulus for RLPs to set in motion a process of change. Second, I consider how movement-oriented party transformations retroact on the movements’ perception of RLPs.  The analysis shows that movement mobilization was an opportunity for the RLPs to emerge from the sidelines and achieve greater recognition. Nonetheless the changes they implemented differed, nor was their transformation equal in its strength and duration. While variation can be observed even over the same case through time, the macro result is that Greek RLPs adopted greater movement-oriented changes that helped them in cultivating stronger ties to social movements than their Italian cousins. The explanation for these differences is found in the combination of the RLPs’ heterodox political culture, higher and constant levels of double membership in both the party and the movements, and social movements’ instrumental attitude towards political institutions.