In this study, we investigated effects of the proportion of language minority students in classrooms on the development of students’ intrinsic motivation in language lessons and the mediating role of instructional climate (e.g., teacher support, focus on student interests). In addition, we explored the interaction between the proportion of language minority students and individual language background affecting students’ motivation. Analyses were based on data collected in 352 German 9th-grade classrooms across 2 measurement points in a multilevel framework. We found that students in classrooms with a higher proportion of language minority students were more intrinsically motivated to learn. However, we found no statistical evidence for the prediction that the compositional effect should be mediated by the instructional climate in the classroom. Cross-level interaction analyses revealed that only language minority students were increasingly motivated in classrooms with a higher proportion of language minority students. Our findings add to the growing evidence indicating that the nature and quality of the social environment are important determinants of school-related motivation.