Background: Childhood overweight and obesity are public health concerns. It is essential to examine the prevalence of overweight and obesity for effective planning and efficient use of resources in the health system. Objectives: The present study’s objective was to investigate the prevalence of overweight and obesity in children aged 8 to 12 years in Tehran, Iran. Methods: In this cross-sectional study, 829 primary school children in Tehran were included. An investigator administered a checklist to collect the relevant data, and anthropometric measurements were performed using standard procedures. We used the World Health Organization’s (WHO) child growth standards to assess overweight and obesity among children. The Chi-square test was used to analyze categorical variables, and statistical significance was confirmed at P < 0.05. Results: Of the students, 366 (44.1%) were boys, and 463 (55.9%) were girls. The prevalence of overweight and obesity were 20.6% and 14.6%, respectively. The prevalence of obesity was higher among boys than among girls (19.4% vs. 10.8%); the highest prevalence of obesity was observed among 9-year-old children. No relationship was identified between the type of school and the prevalence of obesity (P = 0.33). Conclusions: Childhood overweight and obesity were relatively prevalent among 8 - 12-year-old children. So, obesity should be considered a serious health problem in Tehran, requiring more care and interventions to prevent an epidemic of obesity among Iranian children.