The integration of ethnic minority youth can only be successful if they are motivated to establish and maintain social relationships in important institutions such as school. At the same time, worries about negative stereotypes about one's ethnic group can undermine ethnic minority students' motivation to approach others. In the present study, we tested whether social identity threat predicts ethnic minority adolescents' social approach motivation via reduced sense of belonging. We also examined whether multiple social identities (i.e., high endorsement of ethnic and national identity) buffer against the negative effects of social identity threat. In a sample of 426 ethnic minority students from 36 9th-grade classes in Germany, social identity threat was indirectly related to social approach motivation via reduced sense of belonging to the school and class. The interplay of students' ethnic and national identity moderated the relationship of social identity threat and sense of belonging. The relationship was particularly negative for students who endorsed either ethnic or national identity. However, it was less negative for students with integrated multiple social identities and non-significant for students who identified neither with the ethnic nor the national group. Results generalized for social approach motivation towards ethnic majority and minority classmates. These patterns were only found for social approach motivation in face-to-face contact situations, but not in online situations. We discuss these findings in light of the literature on social identity threat and multiple social identities. Practical implications include measures to foster students' sense of belonging and to reduce social identity threat.