Background Obesity during childhood and adolescence is a serious public health problem as it increases the risk of chronic diseases throughout life. Obesity has recently been recognized as a public health threat within low and middle income nations such that research from Thailand has documented an increase in obesity in the past 10 years. While there are several descriptive studies reporting the presence of obesity among adults and children, few have examined relationships between social and dietary factors and obesity in this country. Thus the objective of this study is to examine predictors of overweight/obesity among Thai adolescent girls. Methods This cross-sectional exploratory study uses logistic regression to examine predictors of overweight/obesity among Thai adolescent girls. Anthropometric, dietary, economic, demographic, and activity data were collected from 342 adolescent girls ages 9 to 18 years living in suburban Bangkok, Thailand in 2004 and in 2005. Results The most important predictors of overweight/obese girls included early menarche (p < 0.01), birth order (p < 0.01), sports after school (p < 0.05), and moderate consumption of potato chips (P < 0.05). For instance, girls who were at most 10 years old at menarche were 4.24 (95% CI=1.42, 12.72) times more likely to be overweight/obese than girls who were 12 years old. Conclusions The predictive nature of early menarche has been well documented in studies around the world. However, the other major predictors may be specific to this population in Thailand, and thus may assist in better understanding social determinants of obesity.