BACKGROUND: Screen media use in early childhood is associated with increased risk for health and developmental problems. A preventive intervention during a regular examination in pediatric practices aims to prevent dysregulated screen time by children under three years of age. This protocol describes a process evaluation with the aim of understanding how to implement most effectively a complex intervention to prevent dysregulated screen time in childhood. For this purpose, the process evaluation combines two frameworks to identify both the implementation process and the relevant contextual factors. METHODS: The focus of this process evaluation is a nation-wide, cluster-randomized controlled trial with a complex intervention. Two-thirds of all pediatric practices registered with Professional Association of Pediatricians in Germany received the intervention materials with the option to participate in May 2022, while one-third maintain as usual. Since than an app has been collecting children's screen time and developmental diagnostic parameters using a longitudinal parent survey. The process evaluation will examine the implementation process following the RE-AIM scheme (Reach, Effectiveness, Adoption, Implementation, and Maintenance) as well as the relevant contextual factors influencing the effectiveness of the implementation process following the Consolidated Framework for Implementation Research. Data collection will include quantitative and qualitative methods. Measurement points are at the start of the intervention, several times during the ongoing intervention and after two years. The two groups of people will be interviewed: Pediatricians who use the intervention in their practice and parents who have received the intervention. DISCUSSION: This protocol illustrates the process evaluation of a complex, app-based intervention in pediatric practices. It combines the two frameworks RE-AIM and CFIR and thus covers all aspects of the evaluation of the implementation process of a complex intervention. TRIAL REGISTER NUMBER: DRKS00032258
https://drks.de/search/en/trial/DRKS00032258.