This Finnish longitudinal study investigated the role of relationships with mothers (n = 631) and teachers (n = 56), and adolescents' (n = 848; 53.9% girls, 46.1% boys) temperament in the development of the adolescents' socioemotional functioning during the transition to lower secondary school (Grade 6 to Grade 7; age in Grade 6: M = 12.32, SD = 0.36). The results showed that mother–adolescent closeness predicted a higher level and teacher–adolescent closeness predicted a smaller decrease in prosocial behavior. Teacher–adolescent conflict predicted a higher level, and mother–adolescent conflict predicted an increase in externalizing problems. Only temperament predicted internalizing problems. In addition, relationship quality both mediated and moderated some of the associations between temperament and socioemotional functioning.