The purpose of this paper is to estimate the prevalence of obesity and to identify its potential determinants to optimize the methods of prevention to combat further increases in childhood overweight. The study was carried out on 3444 school children of 6–10 years of age attending 30 schools in northeast Romania. Schools were classified by geographical location and socioeconomic status (SES). Overweight and obesity status were determined using IOTF BMI cut-off points. Prevalence of overweight (including obesity) was found to be 24.6% among boys and 22.6% among girls, whereas the prevalence of obesity was 7.8% in boys and 6.3% in girls. High SES (OR: 1.46; 95% CI: 1.10–1.93) and eating French fries and chips (OR: 1.81; 95% CI: 1.24–2.67) were associated with increased risk of overweight. In high- and medium-SES children, overweight was positively associated with the consumption of French fries and chips (2.93, 95% CI: 1.54–5.60 and 1.82, 95% CI: 1.04–3.21). In low-SES children, overweight was associated with low fruit consumption (0.21, 95% CI: 0.05–1.00) and sedentary behavior (3.37, 95% CI: 1.13–10.05). Therefore, the social and environmental determinants should be considered when constructing and implementing preventive measures regarding overweight and obesity.