Background: Education Outside the Classroom (EOTC) is a teaching method that aims to promote schoolchildren's learning, physical activity (PA), social relations, motivation, and well-being. EOTC activities are characterized by teachers using the local environment in their teaching, and involve innovative teaching methods, child-led approaches to problem-solving, experimentation, cooperation, PA, and play. EOTC has become common practice for many teachers in Scandinavia; however, only case studies have evaluated its impacts. The TEACHOUT study aims to evaluate the impacts of EOTC on Danish schoolchildren's PA, social relations, motivation, well-being, and learning. Methods: TEACHOUT is a quasi-experimental, cross-disciplinary study. Sixteen schools participated, containing 19 EOTC school classes and 19 parallel non-EOTC classes, with a total of 834 children aged 9 to 13 years. Measures of the children's social relations, motivation for school, well-being, and academic performance were collected at the beginning and end of the school year. Data on PA levels were collected over ten-day periods during the school year using accelerometers. The amount and characteristics of the actual EOTC provided in both EOTC and non-EOTC classes were monitored day-to-day throughout the school year, using an online teacher survey platform. The effects of EOTC are mainly analysed by comparing EOTC pupils to non-EOTC (i.e. control) pupils based on their scores on the outcome variables (i.e. school performance, well-being, motivation, and social relations) at the end of the school year, adjusting for the baseline values (from the beginning of the year). The impacts of EOTC on PA are evaluated by comparing the total as well as context-specific amounts of PA of children participating in EOTC to those of children in their parallel non-EOTC classes. Furthermore, the interdependencies between PA, social relations, well-being, motivation, and learning are explored using path analysis. To help describe and understand the processes that have led to the quantitative outcomes, qualitative case observations of children's practices and interactions in EOTC as well as classroom teaching were carried out and combined with qualitative interviews about children's perceptions of these practices. Discussion: The TEACHOUT study represents a holistic multidisciplinary approach to educational and school health-promotion research through its study design and combination of scientific disciplines and methods, as well as its focus on the interdependent relations between learning, PA, social relations, well-being, and motivation. This will result in a comprehensive picture of school health promotion and children's health and well-being, which will broaden the understanding of the potential benefits of EOTC in school health promotion and primary education. These results can be used to inform and guide future policy and practice.