International organizations promoting political liberalization and economic integration have become increasingly contested by some of their own members that do not abide by liberal norms. Yet our knowledge about whether these illiberal actors might change the decision-making process within international organizations remains limited. We argue that as more illiberal domestic parties emerge, liberal majority positions in democratic international organizations face increased contestation. We expect this development to be driven mainly by illiberal parties from liberal democracies. To provide evidence for our claims, we study roll call votes in the Parliamentary Assembly of the Council of Europe (PACE), one of the most powerful international parliaments to date and one committed to promoting liberal values. Leveraging an original dataset recording about 400,000 individual votes cast in PACE decisions, we find that illiberal parties are considerably more likely to cast dissenting votes than liberal parties. In contrast to our theoretical expectations, illiberal parties from illiberal (and not liberal) political systems challenge the majority most often. However, being in government mitigates illiberal parties’ challenging behavior. Our study has implications for the potential threat of emerging illiberal actors to international liberal institutions and organizations.