The quality of peer attachment in adolescence is an important determinant of psychosocial adjustment. To date, few clear conclusions can be drawn about the most important factors associated with the quality of peer attachment. This study aimed to identify the most important individual and relational factors associated with peer attachment quality, and to establish their relative contribution. Early adolescents (N = 634, 45.7% girls), 11.29 years on average at the first measurement time, participated in the longitudinal study. Individual and relational factors were measured at T1; parent and peer attachment quality were measured at T2, 2 years later. First, a multiple linear regression analysis was performed using the global score for peer attachment as the outcome variable. Second, a path model, including the three specific attachment dimensions (trust, communication, and alienation), was tested. The invariance of the models by sex was also examined. The analyses showed that peer attachment was associated positively with the adolescent--parent attachment relationship, the adolescent's age, as well as the teacher--student relationship, and associated negatively with the presence of internalizing behavior problems and social rejection. These variables helped explain 21.3% of the total variance. Give or take a few nuances, these same variables were also related to the three specific peer attachment dimensions. Sex differences were observed for the associated factors of these specific dimensions. This study highlights the importance of certain relational variables, in both family and school contexts, as correlates of peer attachment.