This paper re-contextualizes Karl Popper’s thought within the anti-nationalist cosmopolitan tradition of the Central European intelligentsia. It argues that, although Popper was brought up in an assimilated Jewish Viennese household, from the perspective of the Jewish Enlightenment or Haskalah tradition, he can be seen to be a modern day heterodox Maskil (scholar). Popper’s ever present fear of anti-Semitism and his refusal to see Judaism as compatible with cosmopolitanism raise important questions as to the realisable limits of the cosmopolitan ideal. His inability to integrate an understanding of Jewishness in his cosmopolitan political ideal resulted in his strong opposition to Zionism and the state of Israel. By comparing Popper’s positions with those of Hermann Cohen, another neo-Kantian philosopher, I argue that although their solutions fall short in certain respects, their arguments have continuing purchase in recent debates on cosmopolitanism and the problem of the integration of minority groups. In addition, the arguments of the Jewish Enlightenment thinkers offer important insights for the current debates on minority integration and xenophobia.