The present study aimed to analyze the prevalence of childhood obesity in Brazil and compare it between boys and girls. This systematic review was conducted and reported according to the guidelines outlined in the PRISMA statement. A systematic search of electronic databases, including PubMed, LILACS, and SciELO, was performed in November 2021. Studies that met the following criteria were included: (i) original quantitative studies regardless of their design; (ii) childhood obesity was clearly defined; (iii) prevalence of childhood obesity was reported or it was possible to extract from tables or figures in the text; and (iv) eligible population included children under 12 years of age. A total of 112 articles were included in the systematic review. The prevalence of childhood obesity in Brazil was of 12.2%, being 10.8% in girls and 12.3% in boys. In addition, wide heterogeneity in the childhood obesity prevalence was observed between the states, since it was 2.6% in Pará, while it was 15.8% in Rondônia. Thus, the importance of urgently implementing measures to prevent and treat childhood obesity should be emphasized aiming to reduce obese children and adolescents and future health problems in adult life resulting from this cardiovascular risk factor.