Parents are children's primary role models, are food and physical activity gatekeepers, and create the structure/lifestyle environment within the home. Thus, parents strongly influence children's weight-related behaviors and have the opportunity to cultivate a "culture of health" within the home. Yet, there continues to be a dearth of evidence-based obesity prevention intervention programs, especially for families with children aged 6 to 11 years, commonly called the middle childhood years. The aim of the HomeStyles-2 online learning mode RCT is to determine whether this novel, age-appropriate, family intervention enables and motivates parents to shape their home environments and weight-related lifestyle practices (i.e., diet, exercise, sleep) to be more supportive of optimal health and reduced risk of obesity in their middle childhood youth more than those in the control condition. The RCT will include the experimental group and an attention control group who will engage in a bona fide concurrent treatment different in subject matter but equal in nonspecific treatment effects. The participants will be families with school-age children who are systematically randomly assigned by computer to study condition. The HomeStyles intervention is predicated on the social cognitive theory and a social ecological framework. The RCT will collect sociodemographic characteristics of the participant, child, and partner/spouse; child and parent health status; parent weight-related cognitions; weight-related behaviors of the parent and child; and weight-related characteristics of the home environment. Enrollment for this study will begin mid-2021.This paper describes these aspects of the HomeStyles-2 intervention: rationale; sample eligibility criteria and recruitment; study design; experimental group intervention theoretical and philosophical underpinnings, structure, content, and development process; attention control intervention; survey instrument development and components; outcome measures; and planned analyses.