This study investigates the relationship between the ethnic classroom composition and interethnic attitudes of adolescents of the native majority and several ethnic minorities in the Netherlands, Germany, England and Sweden. It contributes to prior research by examining the underlying theoretical features of contact opportunities and levels of threat across multiple ethnic groups more accurately, using group-specific measures. Based on Intergroup Contact Theory and Ethnic Group Conflict Theory, contrasting hypotheses on how the ethnic classroom composition affects out-group and in-group attitudes of adolescents are tested with multilevel regression analyses. Across ethnic groups and countries, we consistently find a moderate to substantial relation between ethnic classroom composition and interethnic attitudes in line with Intergroup Contact Theory: a relatively larger out-group size, compared to the in-group, relates positively to out-group attitudes. At the same time, in several cases, a relatively larger in-group size relates to more positive in-group attitudes. The findings point to the significance of balanced ethnic classroom compositions for promoting favourable attitudes between multiple ethnic groups – benefitting especially those who face high levels of prejudice from others and those who are prejudiced towards others – without compromising positive in-group attitudes.