This paper examines the effect of Walmart Supercenters, which lower food prices and expand food availability, on household and child food insecurity. Our food insecurity-related outcomes come from the 2001-2012 waves of the December Current Population Study Food Security Supplement. Using narrow geographic identifiers available in the restricted version of these data, we compute the distance between each household's census tract of residence and the nearest Walmart Supercenter. We estimate instrumental variables models that leverage the predictable geographic expansion patterns of Walmart Supercenters outward from Walmart's corporate headquarters. Results suggest that closer proximity to a Walmart Supercenter improves the food security of households and children, as measured by number of affirmative responses to a food insecurity questionnaire and an indicator for food insecurity. The effects are largest among low-income households and children, but are also sizeable for middle-income children.