The relationship between ethnic socialization by parents and peers and ethnic identity development was examined over a seven-year time span in a sample of 116 internationally adopted Korean American adolescents. Parent report data was collected in 2007 (T1) when the adopted child was between 7–13 years old and again in 2014 at ages 13–20 years old (T2). Adolescent report data also was collected in 2014. We examined differences in parent and adolescent reports of parental ethnic socialization at T2, changes in parent reports of ethnic socialization from T1 to T2, and the relationship among ethnic socialization by parents at T1 and T2, ethnic socialization by peers at T2, and ethnic identity exploration and resolution at T2. Results indicated parents reported higher levels of parental ethnic socialization than did adolescents at T2. Parent-reports of parental ethnic socialization also decreased between childhood and adolescence. Adolescents reported higher parental ethnic socialization than peer ethnic socialization at T2. Path analysis demonstrated positive indirect pathways among parental ethnic socialization at T1, parental ethnic socialization and peer ethnic socialization at T2, and ethnic identity exploration and ethnic identity resolution at T2. The study highlights the cultural experiences of transracial, transnational adopted individuals, the role of both parents and peers in ethnic socialization and ethnic identity development, and the importance of longitudinal and multi-informant methodology.