In a changing and complex environment, international sport federations (IFs) have to face new challenges. These challenges can trigger or hinder IFs’ professionalization processes. While researchers have examined organizational change and professionalization of national sport federations (NFs) and clubs, studies on IFs are rare. Considering professionalization as an important element of IFs’ change processes in recent years, the study attempts to fill this gap. The conceptual framework is based on the concepts and dynamics of organizational change, the influence of isomorphic pressures and the operationalization of a multi-level framework. Data from six case studies was analyzed by means of qualitative content analysis. Findings reveal multiple causes of IFs’ professionalization. Three particular findings are discussed: professionalization as a dynamic process with phases of acceleration that vary depending on IFs’ size; IFs’ becoming increasingly business-like through isomorphic changes; and five causes of particular relevance to IFs’ current professionalization process.