Obesity is on the rise in developing countries, especially in urban areas. In this study, a cross-sectional study was conducted in two divisions in Nairobi province to determine the prevalence of and some risk factors associated with overweight and obesity among school children in Nairobi, Kenya. It involved 344 school children aged 9-14 years drawn from four randomly selected public and private primary schools. Weight and height were measured and body mass index was calculated. Nutrition status was determined using the World Health Organization age and gender specific BMI-for-age Z-scores (BAZ). The chi-square test was used to determine the relationship between overweight/obesity and selected socio-demographic characteristics. Complete anthropometric measurements were available for 321 children. Prevalence of combined overweight and obesity (BAZ > +1SD) was 19.0%, with prevalence being higher among girls (21.0%) than boys (16.9%) (p = 0.632). The prevalence among children in private schools was significantly higher (29.0%) than among those in public schools (11.5%) (p = 0.000). In addition, being a single child (p = 0.020), birth order (p = 0.017), mother’s occupation (p = 0.023) and type of residence (p = 0.028) were positively associated with overweight/obesity. The prevalence of overweight/obesity determined in this study is high and is a public health concern, with girls particularly at risk of becoming overweight.