IntroductionRacial/ethnic minority school-age children are at risk for unhealthy weight gain during the summer, and there is a dearth of information regarding the underlying behavioural and environmental factors. The study objective is to provide an in-depth examination of dietary and physical activity behaviours and food, physical activity, and social environments of African American and Hispanic school-age children during the summer.Methods and analysisAn observational study will be conducted using a multistate (Ohio and Indiana, USA) prospective design examining the weight gain trajectory among a racially/ethnically diverse convenience sample of economically disadvantaged school-age children. In addition, a subset of these children will be evaluated to learn their daily health behaviours and food, physical activity, and social environments during the summer. Comparisons will be made between children who routinely attend programming and those who do not, both in the larger sample and subset. Determinants of programme participation and factors that may enhance the beneficial effects of programme participation will also be identified. Data collection at the Indiana site is planned for summer 2018.Ethics and disseminationThis study is approved by The Ohio State University Behavioral and Social Sciences Institutional Review Board. Results from this study will be disseminated in publications for practitioners, scientists and stakeholders.Trial registration numberNCT03010644; Pre-results.