Background: This study adds to the body of evidence regarding the theoretical underpinnings of interpersonal psychotherapy and the mechanisms through which it impacts upon depression in adolescents. Aims: The aims were to determine whether the interpersonal constructs proposed to underpin interpersonal psychotherapy do indeed change in response to this therapy and whether such changes are associated with changes in depression in young people. Method: Thirty-nine adolescents, aged 13–19 years, with a primary diagnosis of major depressive disorder, were randomly assigned in blocks to group or individual treatment. Assessments were conducted at pre and posttreatment, and 12-month follow-up. Results: The results supported the hypotheses, with significant improvements in social skills, social functioning, and the quality of parent-adolescent relationships, and an increase in secure attachment style and decrease in insecure attachment style being evident following treatment. Benefits were maintained at 12-month follow-up. Adolescents who showed greater reductions in depressive symptoms over this period tended to also show greater improvement in parent reported social skills, quality of the parent-adolescent relationship, and attachment style from pretreatment to 12-month follow-up. Conclusions: The findings are consistent with the proposed underpinnings of interpersonal psychotherapy. Adolescents showed significant improvements in interpersonal functioning and changes in attachment style following treatment, and changes in social skills, parent-adolescent conflict and attachment style were associated with reductions in depression. As such, the results add to the body of knowledge regarding the construct validity of interpersonal psychotherapy as an intervention for depression in young people. Clinical implications and directions for future research are discussed.