Background: Management of Type 1 diabetes comes with substantial personal and psychological demands particularly during adolescence, placing young people at significant risk for mental health problems. Supportive parenting can mitigate these risks, however the challenges associated with parenting a child with a chronic illness can interfere with a parent's capacity to parent effectively. Interventions that provide support for both the adolescent and their parents are needed to prevent mental health problems in adolescents; to support positive parent-adolescent relationships; and to empower young people to better self-manage their illness. This paper presents the research protocol for a study evaluating the efficacy of the Nothing Ventured Nothing Gained online adolescent and parenting intervention which aims to improve the mental health outcomes of adolescents with Type 1 diabetes. Method/Design. A randomized controlled trial using repeated measures with two arms (intervention and wait-list control) will be used to evaluate the efficacy and acceptability of the online intervention. Approximately 120 adolescents with Type 1 diabetes, aged 13-18 years and one of their parents/guardians will be recruited from pediatric diabetes clinics across Victoria, Australia. Participants will be randomized to receive the intervention immediately or to wait 6 months before accessing the intervention. Adolescent, parent and family outcomes will be assessed via self-report questionnaires at three time points (baseline, 6 weeks and 6 months). The primary outcome is improved adolescent mental health (depression and anxiety). Secondary outcomes include adolescent behavioral (diabetes self-management and risk taking behavior), psychosocial (diabetes relevant quality of life, parent reported child well-being, self-efficacy, resilience, and perceived illness benefits and burdens); metabolic (HbA1c) outcomes; parent psychosocial outcomes (negative affect and fatigue, self-efficacy, and parent experience of child illness); and family outcomes (parent and adolescent reported parent-adolescent communication, responsibility for diabetes care, diabetes related conflict). Process variables including recruitment, retention, intervention completion and intervention satisfaction will also be assessed. Discussion. The results of this study will provide valuable information about the efficacy, acceptability and therefore the viability of delivering online interventions to families affected by chronic illnesses such as Type 1 diabetes. Trial registration. Australian New Zealand clinical trials registry (ANZCTR); ACTRN12610000170022.