Intergroup trust is a crucial psychological foundation for promoting ethnic unity and maintaining social stability. This study explored the dynamic relationship between intergroup contact and trust among adolescents in ethnic regions of China and sought to uncover the mechanisms behind this relationship. Through a two-wave longitudinal survey of 679 adolescents, employing the Intergroup Contact Experience Scale, the Intergroup Trust Scale, and the UCLA Loneliness Scale, the study yielded several findings: (1) Over the year, a significant increase in intergroup contact was observed among the adolescents. Notably, junior high students demonstrated a marked rise in intergroup trust, whereas no significant change was discernible in high school students. (2) Cross-lagged analysis indicated that early intergroup contact significantly predicted subsequent intergroup trust and loneliness. Initial loneliness also forecasted future intergroup trust, yet early intergroup trust did not predict later intergroup contact or loneliness. (3) Loneliness served as a partial mediator in the longitudinal link between intergroup contact and trust among these adolescents. These findings reinforce the premise that in China’s ethnic regions, intergroup contact is a precursor to intergroup trust, both directly enhancing trust among adolescents and indirectly by reducing loneliness.