This study included 11,825 participants of a Spanish dynamic prospective cohort based on former students from University of Navarra, registered professionals from some Spanish provinces, and university graduates from other associations, followed-up for 6.1 years. We aimed to assess the association between childhood or young adult overweight/obesity and the risk of depression. Participants were asked to select which of nine figures most closely represented their body shape at ages 5 and 20 years. Childhood and young adult overweight/obesity was defined as those cases in which participants reported body shape corresponding to the figures 6-9 (more obese categories) at age 5 or 20, respectively. A subject was classified as incident case of depression if he/she was initially free of depression and reported physician-made diagnosis of depression and/or the use of antidepressant medication in at least one of biannual follow-up questionnaires. The association between childhood and young adult overweight/obesity and incidence of depression was estimated by multiple-adjusted hazard ratio (HR) and its 95% confidence interval (95% CI). Overweight/obesity at age 5 years predicted an increased risk for adult depression (HR = 1.50, 95% CI = 1.06-2.12), and a stronger association was observed at age 20 years ((HR = 2.22, 95% CI = 1.22-4.08), (subjects younger than 30 years at recruitment were excluded from this last analysis)). Childhood or young adult overweight/obesity was associated with elevated risk of adult depression. These results, if causal and confirmed in other prospective studies, support treating childhood and young adult overweight/obesity as part of comprehensive adult depression prevention efforts.