BACKGROUND: Depression and anxiety commonly emerge during adolescence and are associated with substantial personal, social, and economic burden. Parenting practices are well-established modifiable risk and protective factors for adolescent affective disorders, yet many parents lack access to evidence-based guidance. Digital parenting interventions offer a potentially scalable approach, but evidence for their effectiveness in high-risk adolescent populations remains limited. METHODS: The PIPA Trial was a school-based, UK-wide, two-arm randomised controlled trial evaluating a personalised online parenting programme compared with a standard educational parenting package. Parent-adolescent dyads were eligible if adolescents aged 11-15 years scored ≥7 on the Short Mood and Feelings Questionnaire (SMFQ). The primary outcome was change in parent-reported adolescent depressive symptoms (SMFQ) from baseline to 15 months. Secondary outcomes included parenting practices, parent-child attachment, adolescent anxiety, wellbeing, and parent wellbeing. RESULTS: A total of 512 dyads were randomised, with 510 included in intention-to-treat analyses. Adolescents presented with elevated baseline depressive symptoms and substantial prior mental health difficulties. Depressive symptoms declined over time in both trial arms, with no significant between-group difference at 15 months (adjusted difference - 0.12 SMFQ points, 95% CI -1.23 to 0.98). In contrast, parents allocated to the personalised programme showed greater improvements in guideline-concordant parenting practices and parent-child attachment. Engagement was modest, and over 40% of adolescents triggered duty-of-care procedures due to elevated risk during the trial. CONCLUSIONS: In this high-risk UK sample, a personalised digital parenting programme improved parenting practices but did not reduce adolescent depressive symptoms beyond an active educational control. These findings highlight the importance of matching intervention intensity to baseline symptom severity and support the role of digital parenting programmes as part of stepped prevention and early intervention frameworks. TRIAL REGISTRATION: ISRCTN63358736.