Interactions with classroom friends may be an important contributor to first and second language development, but to date this hypothesis has not been tested. Using a longitudinal design, the current study investigated the relationship between classroom friendships and oral language development in children. In 8 classrooms, we assessed the relationship between oral language skills and classroom social networks. Across the classrooms, 165 primary school children in Austria (83 boys; 119 L2 learners; age: 6-10) were assessed on oral language proficiency at the beginning of the school year (T1) and 6-7 months later (T2). Results indicated that the more reciprocal best friendships at T1, the greater language improvement at T2. Language improvement was strongest among friends with moderate differences in language proficiency, regardless of whether students were first or second language learners. These results underline the importance of positive social relations at school for language learning broadly.