The Labour and Socialist International (LSI) was a major vehicle for transnational socialist cooperation during the interwar years and thus seemed to continue the traditions of socialist internationalism. In the realm of international relations, however, it championed key tenets of liberal internationalism. The LSI supported the idea of a League of Nations and embraced the notion of a world order based upon democratic nation-states. While it criticised some aspects of the international system, its overall emphasis was on reform rather than revolution. The article sheds light on the wider phenomenon of interwar internationalism by tracing the LSI's relationship with the League of Nations, with the politics of peace more generally and with the competing internationalism of the communists.