Research in the US indicates that classroom diversity is related to better social adjustment of students, but research on this association in European classrooms is limited in scope and yields inconsistent findings. This study examined how classroom ethnic diversity is related to social adjustment of societally dominant versus minoritized ethnic groups, and how an open classroom climate for discussion contributes to this. This was examined in low to moderately diverse Dutch classrooms (2703 secondary school students, from 119 classrooms and schools, Mage = 14, 50% female, 18% foreign-born parents). Results revealed that students from minoritized groups reported lower social adjustment. For all students, classroom ethnic diversity was related to worse social adjustment which was partly explained by classroom socioeconomic status (SES). An open classroom climate for discussion did not moderate the relation between diversity and social adjustment. The findings indicate that students’ social adjustment is worse in ethnically diverse and low-SES classrooms, and an open classroom climate for discussion does not solve this.