Purpose: Evidence suggests that the construct of ethnic identity (EI) in adolescents changes over time and is related to factors including social connectedness, social competence, coping, and self-worth. Transitional changes of EI in middle adolescence have not been studied. Method: This analysis, part of a longitudinal study of health behaviors, explored how EI changes from the first year of high school to the last, and it examined relationships among EI, social connectedness, indicators of social competence, coping, and self-worth in 602 youth (49% Hispanic, 59% female). Results: EI was significantly related to social connectedness, social competence, coping, and self-worth as adolescents entered and prepared to exit high school. EI increased significantly over time for both genders and for Hispanics more so than for non-Hispanics. Discussion: Findings support and extend the conceptual framework and previous research. Socialization with one’s ethnic group is associated with developing competence and self-worth. EI development continues throughout high school. Conclusion: Findings extend our understanding of the salience of EI in adolescents in transition throughout high school. Implications: Strengthening EI may contribute to adolescents’ feelings of self-worth, skills in coping, and competence within an ethnically diverse society. Nurses might plan interventions to strengthen EI throughout adolescence.