Research on social movements has traditionally addressed issues of movement emergence and mobilisation, paying little attention to their outcomes and consequences. Moreover, despite research on the political consequences of social movements accelerating in recent years, much has been left under researched, no more so than the impact social movements have on one of the most important actors in liberal democracies: political parties. This paper extends social movement research by examining whether social movements have an impact on political parties and under what conditions impact is more likely to take place. The empirical analysis, investigating whether anti-austerity social movements have had an impact on social democratic parties in Ireland and Spain during the years 2011 to 2016, suggests that the relationship between social movements and political parties is both under-theorised and under-researched, and mistakenly so. The paper finds that while parties are more likely to be influenced by social movements when certain conditions are present, social movements can also have unintended impacts on parties. Ultimately, this paper encourages research on political parties, and particularly research on party change, to pay greater attention to social movements and for social movement research to pay greater attention to political parties.