Early neurodevelopmental risks, compounded with traumatic medical experiences, contribute to emotional and behavioral challenges in as many as 1 in 2 children with congenital heart disease (CHD). Parents report a strong need for supports; yet, there remains a lack of accessible, evidence-based behavioral interventions available for children with CHD and their families. I-InTERACT-North is a web-based stepped-care mental health program designed to support family well-being and reduce behavioral concerns through positive parenting for children with early medical complexity. In previous pilot studies, the program was effective in increasing positive parenting skills and decreasing child behavior problems, with high parent-reported acceptability. This paper presents the protocol for the first randomized study of stepped-care parent support for families of children with CHD. This study will involve a single-site, 2-arm, single-blind randomized controlled trial to evaluate (1) the feasibility and acceptability of a web-based stepped-care parent support program (I-InTERACT-North) and (2) the effectiveness of the program in enhancing positive parenting skills and reducing behavioral concerns among families of children with CHD. Families will be randomized (1:1) to either receive treatment or continue with care as usual for 12 months. Randomization will be stratified by child's sex assigned at birth and baseline parent-reported child behavior intensity. Primary outcomes include positive parenting skills and child behavior at baseline, 3 months, 6 months, and 12 months. Secondary outcomes include parental mental health, quality of life, service usage, and feasibility including program reach and adherence. A sample size of 244 families will provide >95% power to detect an effect size of d=0.64. Based on attrition data from pilot studies, a target of 382 families will be enrolled. Parent reports of acceptability, adoption, and suggested adaptability of the program will be examined using cross-case thematic analyses. Primary efficacy analysis will be conducted using an intent-to-treat approach. Generalized estimating equations will be used to examine changes in positive parenting. Child behavior, quality of life, and parent mental health will be tested with repeated-measures analyses. Additional sensitivity and replication analyses will also be carried out. Recruitment began in February 2024, and recruitment and follow-up will continue until January 2029. We anticipate results in late 2029. This study aims to test the effectiveness of I-InTERACT-North web-based stepped-care parent support in improving positive parenting skills and reducing child behavior problems in families of children with CHD compared with a care as usual control group. Results will inform future clinical implementation and expansion of this program among families of children with early medical conditions. ClinicalTrials.gov NCT06075251; https://clinicaltrials.gov/study/NCT06075251. DERR1-10.2196/64216.