The relationship between weight and sexual behavior among adolescents is poorly understood. We examined this relationship in a nationally representative sample of high school girls. We performed a cross-sectional analysis of self-reported data from 7193 high school girls who completed the 2005 Youth Risk Behavior Surveillance survey. We used multivariate logistic regression to examine associations among 3 weight indices (BMI calculated from self-reported weight and height, perceived weight, and weight misperception) and 6 sexual behaviors (ever had vaginal sex; sex before age 13; >or=4 sexual partners; and alcohol, condom, and oral contraceptive use at last sex) adjusting for age, race/ethnicity, and a history of intimate partner violence. There were no differences in the likelihood of ever having sex on the basis of BMI or weight perception accuracy; however, girls who perceived themselves as overweight were less likely to have ever had sex. Among sexually active girls, those who had low BMI and perceived themselves as overweight or had overweight misperceptions were less likely to report condom use at last sex. Sexually active girls who perceived themselves as overweight were also more likely to have had sex before age 13. Associations between the 3 weight indices and sexual risk behaviors varied across racial/ethnic groups. Sexual risk behaviors may be more common among girls who are underweight or perceive themselves (accurately or not) to be overweight and vary by racial/ethnic group. This suggests that girls at weight extremes and those from different racial backgrounds may have unique sexual health education and prevention needs.