ObjectivesThis study aims to assess the current state of childhood overweight and obesity in Spain, and its relationship with socioeconomic status and health-related behaviors. MethodsPopulation-based cross-sectional observational study, based on the 2017 National Health Survey in minors in Spain. This study included all children surveyed who were aged 1–14 years. Childhood obesity was estimated from the z-score of the body mass index. ResultsThe study included 4882 children aged 1–14 years (mean 7.5). The prevalence of obesity was 18.6% (95% confidence interval [CI] 18.5–18.7), while 13.5% (95% CI 13.4–13.6) were overweight. These figures represent over a million children in Spain who are obese and nearly 750,000 who are overweight. A north–south geographic gradient was apparent, with higher prevalence of unhealthy body weight in southern Spain. Factors associated with childhood obesity were low socioeconomic status, poor diet and sedentarism, among others. ConclusionsChildhood overweight in Spain is strongly associated with socioeconomic status and other factors such as diet and sedentarism. Multidisciplinary public health interventions are needed to reduce this serious health problem in children.