Background: Childhood obesity is a serious global health concern that affects around 20% of children worldwide. Digital health behaviour change interventions have the potential to improve behaviours that can contribute to childhood obesity, such as diet and physical activity, but often lack sufficient user engagement to achieve significant impact. Objective: The aim of this project is to develop evidence to better understand how users engage with digital interventions and how Behaviour Change Techniques can be leveraged to support engagement. Specifically, the study will examine the impact of a family-focused app for childhood obesity prevention on health behaviours, health outcomes, and communication between families and healthcare professionals. Methods: A pre-post, mixed-methods evaluation will examine the impact of the NoObesity app on families’ physical activity and dietary behaviours and on healthcare professionals’ self-efficacy at communicating with families about childhood obesity. Secondary outcomes will include well-being, usability, and users’ engagement with and perceptions of the intervention. An initial sample of 1000 families (children and young people of any age and weight and their parents) and 180 healthcare professionals will be recruited to participate in the study, a subset of whom will be invited to qualitative semi-structured interviews. The study implementation and follow-up period will last for 6 months, with the outcomes measured at baseline and 3- and 6-months post-baseline. Quantitative outcomes will be compared over time using repeated-measures ANOVA and qualitative data will be analysed thematically and triangulated with app use data. Results: Ethical approval was received from Newcastle University Faculty of Medical Science Ethics Committee (2688/41816) on 22 March 2024. Recruitment has not yet begun. Conclusions: The project’s key contributions will be to generate evidence of potential for a family-based digital intervention to support families’ health behaviour change and healthcare professionals’ confidence in their ability to support them and to improve our understanding of how particular Behaviour Change Techniques can be used to support engagement with the intervention and its target behaviours. Findings will be disseminated via peer-reviewed journal and to the general public with support from patient and public involvement representatives.