Insomnia and attention deficit hyperactivity disorder (ADHD) are highly comorbid conditions with an intricate, bidirectional relationship. In particular, insomnia and ADHD often co-occur, with 22.9-34.6% of adolescents with insomnia having clinically significant ADHD, and 22-33.5% of adolescents with ADHD experiencing comorbid insomnia symptoms. Insomnia is not only associated with increased ADHD severity, but also linked to an increased risk for other mental health problems and poorer treatment response in young people with ADHD. Behavioural sleep interventions have shown promise in improving sleep in young children with comorbid ADHD and insomnia. Nonetheless, when treating adolescent insomnia, especially those with ADHD, there is a need to consider the unique sleep and circadian features as well as psychosocial factors associated with not only ADHD but also adolescence, a challenging developmental stage. To date, there is a paucity of research to test the efficacy of cognitive behavioural therapy for insomnia (CBT-I) in adolescents with comorbid ADHD and insomnia. To address the limitations in the existing literature, this study aims to conduct a randomised controlled trial (RCT) to examine the effects of CBT-I relative to a usual care (UC) condition in adolescents with ADHD, on improving sleep and ADHD symptoms, and other clinical and daytime symptoms as well as overall functional improvement.