In our current diverse society, young people need a wide scope of social citizenship competences to coexist with and understand the otherness of others. Schools are one of the social practices in which students develop these social citizenship competences. This study aims to identify how socio-ethnic diversity in the classroom is related to the social citizenship competences of sixth grade primary school students. To measure students' social citizenship competences, a video-based situational judgement test (skills) and the Citizenship Competences Questionnaire (knowledge, attitudes and reflection) were administered to 791 students in 54 classes in Flanders (the northern part of Belgium). Socio-ethnic classroom diversity variables were measured by means of a student and parent background questionnaire. A regression analysis with a two-level design at student and classroom level was conducted for each of the social citizenship scales. The results indicate that differences in students' social citizenship competences are mainly explained by student-level variables. Variables measuring socio-ethnic diversity in the classroom showed that students in more social and ethnic diverse classrooms had more positive attitudes towards dealing with differences, whereas no significant relation with other social citizenship scales was found.