To determine whether proximity to a supermarket modified the effects of an obesity intervention. We examined 498 children aged 6 to 12 years with a body mass index (BMI) at or above the 95th percentile participating in an obesity trial in Massachusetts in 2011 to 2013. The practice-based interventions included a computerized clinician decision support plus family self-guided behavior change or health coaching. Outcomes were 1-year change in BMI z-score, sugar-sweetened beverage intake, and fruit and vegetable intake. We examined distance to the closest supermarket as an effect modifier. Distance to supermarkets was an effect modifier of 1-year change in BMI z-score and fruit and vegetable intake but not sugar-sweetened beverage intake. With each 1-mile shorter distance to a supermarket, intervention participants increased their fruit and vegetable intake by 0.29 servings per day and decreased their BMI z-score by -0.04 units relative to controls. Living closer to a supermarket is associated with greater improvements in fruit and vegetable intake and weight status in an obesity intervention. (Am J Public Health. Published online ahead of print January 21, 2016: e1-e6. doi:10.2105/AJPH.2015.302986).