This study aims to explore the interrelatedness of socio-ethnic classroom diversity, teachers' citizenship beliefs, teacher practices to create a citizenship-fostering classroom climate, and their relationship with social citizenship competences of primary school students in Flanders. Data were gathered from 686 sixth-grade primary school students in 44 classrooms and their 44 teachers. To analyze the data, a multilevel path analysis was conducted for each of the social citizenship components (i.e., skills, knowledge, attitudes, reflections). Results indicate that ethnic classroom diversity is related to teachers' liberal-critical citizenship beliefs, which in turn are linked to their practices in creating a citizenship-fostering classroom climate. Regarding their role in fostering students' social citizenship competences, three significant relationships were found. First, more social classroom diversity was positively related to students' attitudes toward dealing with conflicts. Second, teachers' conservative citizenship beliefs negatively related to students' reflections about acting in a socially responsible manner and dealing with differences. Finally, contrary to expectations, students from classrooms with teachers contributing more to a citizenship-fostering climate scored lower on knowledge about acting in a socially responsible manner and dealing with conflicts.