Comorbidity between AOD and mental health disorders in young people is common, and current treatment approaches often fail to adequately address the risk and protective factors underlying these disorders. Transdiagnostic cognitive behavioural therapy (CBT) cuts across diagnostic boundaries to target the common psychological factors that underlie multiple disorders. Despite this, research is yet to determine the efficacy of transdiagnostic CBT treatments for young people with comorbid disorders. This pilot randomised controlled trial aims to examine the feasibility, efficacy, and cost-effectiveness of a new risk and protective factor-targeted transdiagnostic CBT telehealth program (FullFix) for comorbid AOD and mental health problems in young people. Moderators and mediators of treatment outcomes will also be examined. This highly novel project will be the first to utilise a transdiagnostic intervention to target both risk and protective factors for AOD and mental health in young people. Moderater and mediators of treatment outcomes will also be examined.