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Objective: We examined BMI development across changes in the built environment during the transition from adolescence to young adulthood and explored the moderating role of genetic risk., Methods: We used longitudinal data from individuals aged 16 to 25 years in the TRacking Adolescents' Individual Lives Survey (TRAILS) that we linked to built environment data for 2006, 2010, and 2016 from the Geoscience and Health Cohort Consortium (GECCO). We fitted a latent growth model of BMI and examined associations of changes in fast-food restaurant density and walkability with changes in BMI (n = 2735), as well as interactions of changes in fast-food restaurant density and walkability with genetic risk (n = 1676)., Results: Changes in fast-food restaurant density (e.g., Δ2010-2006: β = -0.04, 95% CI: -0.11 to 0.03) and walkability (e.g., Δ2010-2006: β = -0.05, 95% CI: -0.14 to 0.05) were not associated with BMI changes. Additionally, genetic risk did not moderate these associations., Conclusions: We found limited evidence that moving to neighborhoods with higher fast-food restaurant density or less walkability was associated with BMI changes or that genetic risk moderated these associations. Our findings suggest that associations between the built environment and BMI changes during the transition into young adulthood are likely small., (© 2025 The Author(s). Obesity published by Wiley Periodicals LLC on behalf of The Obesity Society.)