This study investigated the effects of international mobility on adolescent identity development and explored the mediating role of changes in social relationships. We implemented a prospective comparative study design including 457 sojourners (German high school exchange students) and 284 non-sojourners (German adolescents who remained in Germany; 73.3% female; M[subscript age] = 15.63, SD = 0.78), and measured students' identity commitment and reconsideration on 3 occasions over a period of 18 months. The measurements were taken 6 weeks prior to departure (Time [T]1), 7 months after the sojourn experience had begun (T2), and 7 months postreturn (T3), thereby covering both central phases in the context of international sojourning: the transition abroad and the reentry transition. Two identity domains (i.e., Friends and Home) were examined. Longitudinal latent change analyses indicated both transition-specific and domain-specific identity processes. Sojourners showed a relative decrease in Friends commitment and a corresponding increase in Friends reconsideration over time. Although pronounced increases in Home commitment were found for sojourners during the transition abroad, their Home commitment declined throughout the reentry transition. Inverse trends were recorded for sojourners' Home reconsideration trajectories. Multiple mediation analyses revealed sojourn-related changes in subjective closeness toward parents but not friends as a mechanism that partially explained the effects of international mobility on identity development. For future directions, we discuss the role of life events and social relationships as inalienable contexts for identity dynamics.