What is the relationship between unionization and job satisfaction? Despite a great deal of research over several decades, the answer to this question is still uncertain. In contrast to earlier work, which analyzed mostly data from individual companies or countries, we examine the association between union membership and job satisfaction in a cross-national perspective. We therefore combine large-scale survey data with country-level information about union and labor-market characteristics. Our multilevel approach allows us to examine whether and why the unionization–job satisfaction relationship varies across countries. The main finding of our analyses is that the relationship between union membership and job satisfaction varies across countries and that unions matter only for certain aspects of job satisfaction—those that can more readily be changed by unions. This effect, moreover, is contingent on countries’ industrial relations systems, in particular union density, bargaining coverage, and the centralization of bargaining agreements. Taken together, our results show that in order to understand how unionization influences job satisfaction, it is important to distinguish between the various aspects of job satisfaction and to consider the larger context in which unions operate.