Objective: This study evaluated the effectiveness of the Parents Plus Special Needs (PPSN) programme, a seven-week parenting group intervention for parents of adolescents with intellectual disabilities. Method: In a cluster randomised controlled trial, 24 intellectual disability services supporting families of adolescents with an intellectual disability were assigned to PPSN (12 services; 141 parents) or waitlist control group (12 services; 136 parents). Primary outcomes were parent-reported parenting practices, family adjustment, problem behaviours, emotional problems, and prosocial behaviours. Secondary outcomes were parental satisfaction, parental self-efficacy, and goal attainment. Results: Compared to the waitlist group, participants in the PPSN group reported improvements in parenting practices, problem behaviours, parental satisfaction, parental self-efficacy and goal attainment, which were retained at 3-month follow-up. There were additional gains for family adjustment at follow-up. Conclusion: The PPSN is effective in improving parenting behaviour, family relationships, and problem behaviours in adolescents, but not in improving emotional difficulties.