There is growing support for the large-scale implementation of parenting programs for the prevention of child behavior disorders and child maltreatment in younger children. However, there is only limited evidence on the efficacy of parenting programs in modifying risk and protective factors relating to adolescent behavior problems. This study examined the efficacy of Group Teen Triple P (GTTP), an eight-session parenting program specifically designed for parents of young adolescents. Seventy-two families with adolescents aged between 12 and 15 years were randomly assigned to either GTTP (n = 35) or a care as usual (CAU) control condition (n = 37). Compared to CAU parents, parents who received GTTP reported significant improvements in parenting practices, parenting confidence, the quality of family relationships, and fewer adolescent problem behaviors at post-intervention. Several of the parent-reported effects were corroborated by reports from adolescents, including decreases in parent–adolescent conflict and increases in parental monitoring. Adolescents whose parents participated in GTTP also reported significantly fewer behavioral problems than adolescents in the CAU condition. Many of these improvements were maintained at 6-month follow-up.