Background: Capturing voices of young people on issues that affect them is crucial for developing effective, relevant public health policies. We report the use of group model building with young people to identify their views on drivers of adolescent obesity. This is the first stage of the Co-Create project, designed to develop—with adolescents—potential interventions to reduce obesity prevalence. Methods: Co-Create uses an innovative, participatory, multi-staged methodology, underpinned by a systems approach. We followed a group model building script to generate causal loop diagrams, with 16-18-year-olds from three UK schools (mean number of participants per school 14), of factors perceived to contribute to diet and physical activity and, therefore, obesity. Schools were recruited using a sampling frame to represent three quartiles from the index of multiple deprivation ranking of local authorities. Findings: All causal loop diagrams showed that participants identified advertising, the low cost of and easy access to unhealthy food, social media, lack of physical activity, and stress, body image, and other mental health factors as drivers of diet and physical activity and, therefore, obesity. There were no notable differences in themes between the causal loop diagrams. Interpretation: Use of group model building, a participatory systems methodology, captures obesity drivers pertinent to young people, the ways these drivers are intertwined, and potential leverage points for action. This process creates a helpful abstraction from individual behaviour by drawing out system-level, rather than individual, drivers of obesity, although findings are limited to the perceptions of participants and these results represent only urban environments. The results from this first stage of the Co-Create project help generate system-level hypotheses on policy responses to obesity that resonate with young people. In subsequent Co-Create workstreams, adolescents will be recruited to explore interventions to be discussed with policy makers and other stakeholders. The potential effects of the co-created, obesity-related interventions will be simulated using systems dynamics modelling. Funding: EU Horizon 20/20 research and innovation programme for Sustainable Food Security (grant agreement 774210).