The aim of this study was to compare the prevalence of overweight in only children to those with siblings and to explore potential behavioral mediating factors. This study relies upon cross-sectional data collected at survey centers in eight European countries participating in Identification and prevention of Dietary- and lifestyle-induced health EFfects In Children and infantS (IDEFICS). The present analysis is based on measured anthropometry and parent or guardian-reported socio-demographic characteristics. Subjects include 12 720 children aged 2–9 years for whom number of siblings was known. Singletons were more likely (odds ratio 1.52, 95% confidence interval (CI):1.34–1.72) to be overweight than their peers with siblings when controlling for factors related to childhood overweight, including survey country, parental education, parental weight, maternal age, child's age, birth weight and gender. The three southernmost countries have over threefold risk of overweight, dominated by Italy, compared with the north-central countries, which is not explained by the prevalence of singleton children. The excess risk of overweight among children without siblings was robustly observed even when considering behavioral mediating factors (playtime, screen time per day, dietary propensities for sugar or fat, parental attitudes towards food rewards and television in the child's bedroom). Among singletons aged 6–9 years, the excess risk of overweight was 1.70 (95% CI: 1.44–2.01) compared with 1.32 (95% CI: 1.10–1.60) in younger singletons.