This prospective study examined romantic partner selection and socialization among a sample of 78 young adolescents (6th–8th graders). Independent assessments of adolescent and romantic partner adjustment were collected before and after relationships initiated via peer nomination and self-report. Prior to their relationship, adolescents and partners were significantly alike on popularity, physical attraction, and depressive symptoms. Controlling for initial similarity, partners’ popularity, depressive symptoms, relational aggression, and relational victimization significantly predicted changes in adolescents’ functioning in these areas over time. However, the magnitude and direction of change varied according to adolescents’ and partners’ prerelationship functioning. In general, adolescents who dated high-functioning partners changed more than those who dated low-functioning partners, and partner characteristics predicted greater change among low- versus high-functioning adolescents. Results were consistent even when controlling for best friend characteristics. The current findings are among the first to demonstrate unique contributions of romantic partner characteristics to adolescents’ psychosocial functioning.