BackgroundTo meet the scientific and political call for effective prevention of child and youth mental health problems and associated long-term consequences, we have co-created, tested, and optimized a transdiagnostic preventive parent-training intervention, Supportive parents – coping kids (SPARCK), together with and for the municipal preventive frontline services. The target group of SPARCK is parents of children between 4 and 12 years who display symptoms of anxiety, depression, and/or behavioral problems, that is, indicated prevention. The intervention consists of components from various empirically supported interventions representing different theorical models on parent–child interactions and child behavior and psychopathology (i.e., behavioral management interventions, attachment theory, emotion socialization theory, cognitive-behavioral therapy, and family accommodation intervention). The content and target strategies of SPARCK are tailored to the needs of the families and children, and the manual suggests how the target strategies may be personalized and combined throughout the maximum 12 sessions of the intervention. The aim of this project is to investigate the effectiveness of SPARCK on child symptoms, parenting practices, and parent and child stress hormone levels, in addition to later use of specialized services compared with usual care (UC; eg. active comparison group).MethodsWe describe a randomized controlled effectiveness trial in the frontline services of child welfare, health, school health and school psychological counselling services in 24 Norwegian municipalities. It is a two-armed parallel group randomized controlled effectiveness and superiority trial with 252 families randomly allocated to SPARCK or UC. Assessment of key variables will be conducted at pre-, post-, and six-month follow-up.DiscussionThe current study will contribute with knowledge on potential effects of a preventive transdiagnostic parent-training intervention when compared with UC. Our primary objective is to innovate frontline services with a usable, flexible, and effective intervention for prevention of childhood mental health problems to promote equity in access to care for families and children across a heterogeneous service landscape characterized by variations in available resources, personnel, and end user symptomatology.Trial registrationClinicalTrials.gov ID: NTCT05800522