Well implemented, universal parental support is often effective in families with younger children, but research on their effects on families with adolescent children is scarce. In this study, a trial of the universal parent training intervention "Parent Web" in early adolescence is added to the social emotional learning intervention Promoting Alternative Thinking Strategies (PATHS®), completed in early childhood. The Parent Web is a universal online parenting intervention based on social learning theory. The intervention aims to promote positive parenting and family interaction through five weekly modules completed over 6-8 weeks. The main hypothesis is that participants in the intervention group will exhibit significant pre- to post- intervention-related benefits relative participants in the comparison group. The aims of this study are: 1) provide Parent Web as a booster aimed at improving parenting support and practices at the transition into adolescence to a cohort of parents whose children have previously participated in preschool PATHS, and 2) examine the effects of the universal edition of Parent Web. The study has a quasi-experimental design with pre- and post-testing. The incremental effects of this internet-delivered parent training intervention are tested in parents of early adolescents (11-13 years) who participated in PATHS when 4-5 years old compared to a matched sample of adolescents with no prior experience of PATHS. The primary outcomes are parent reported child behavior and family relationships. Secondary outcomes include self-reported parent health and stress. The proposed study is one of the few trials to test the effects of universal parental support in families of early adolescents and will therefore contribute to the understanding of how mental health in children and young people can be promoted across developmental periods through a continuum of universal measures. Trial registration: Clinical trials.gov (NCT05172297), prospectively registered on December 29, 2021.