Childhood overweight and obesity have increased alarmingly in recent decades all over the world, particularly in middle-income countries like Brazil, Mexico and China. In response to the obesity epidemic, several states and governments have introduced restrictions on sales of high-calorie low-nutrient-dense foods and beverages in schools. However, most school canteens around the world continue to offer such unhealthy products. The lack of clear evidence about the impacts of junk food/beverage availability on childhood overweight potentially contributes to delaying the application of regulatory policies. In fact, sales of junk food represent an important source of revenues for schools, especially in contexts of budgetary pressure. Based on a representative sample of Brazilian middle school students, this article takes advantage of local initiatives that began in 2001 aimed at banning sales of junk food and beverages in schools. Among other effects, instrumental variables estimates show that in-school soft drink availability increases male BMI and overweight risk. As expected, the impacts tend to be stronger on non-poor students. No effect was found for girls, probably because of voluntary substitutions with healthier foods to control total calorie intakes and limit weight gain. Alarmingly, in-school junk food/beverage availability is positively correlated with overall junk food/beverage consumption and negatively correlated with overall healthy food intakes. In conclusion, this article provides clear evidence that banning sales of unhealthy products in schools is a useful tool to fight against the worldwide increase in childhood overweight, even in middle-income countries.