We leverage exogenous mixed-dormitory room assignments between Hong Kong/Macau (HM) students and mainland Chinese students at a Chinese university to investigate intergroup contact's effects on national identity. We find that mixed-room experience significantly enhances the national identity with China for both HM and mainland students. The heterogeneous analysis suggests that for HM students, the national identity rises more when paired with culturally proximate mainland peers (Cantonese speakers), and for mainland students, their national identity is bolstered when living with relatively low economic status HM peers. Additionally, mixed-room experience increases the mainland Chinese media usage and the preference for working in the mainland of China after graduation for HM students.