Abstract Background There is limited data on risk factors associated with childhood overweight and obesity in Ghanaian school children. Therefore, the aim of this study was to assess the prevalence of overweight and obesity and associated risk factors in Ghanaian school children. Methods Data for this study were obtained from a cross-sectional survey of 543 children aged 8 and 11 years, attending private and public primary schools in the Adentan Municipality of Greater Accra Region, Ghana. Anthropometric, dietary, physical activity, sedentary behaviours, sleep duration and socio-demographic data were collected. BMI-for-age Z-scores were used to classify children as overweight/obesity. Multivariable logistic regressions were used to assess the determinants of overweight and obesity. Results The overall prevalence of overweight/obesity was 16.4%. Children living in middle (OR = 1.88; 95% CI = 1.01–3.50) and high socioeconomic status (SES) households (2.58; 1.41–4.70) had increased odds of being overweight or obese compared to those living in low SES household. Attending private school (2.44; 1.39–4.29) and watching television for more than 2 h each day (1.72; 1.05–2.82) were significantly associated with increased likelihood of overweight and obesity. Children who slept for more than 9 h a night (0.53; 0.31–0.88) and walked or cycled to school (0.51; 0.31–0.82) had lower odds of being overweight or obese. Conclusions A number of modifiable risk factors were associated with overweight and obesity in this study. Public health strategies to prevent childhood obesity should target reduction in television watching time, promoting active transport to and from school, and increasing sleep duration.