Background: Recent surveys suggest that adolescents and young adults in Southeast Asian nations are at great risks of sexual reproductive health issues. This study explored factors associated with risky sexual behavior (RSB) among unmarried most-at-risk young people in Cambodia. Methods: A two-stage cluster sampling method was used to select 1,204 boys and 1,166 girls aged 10-24 from 252 hotspots in the capital city and seven provinces. A five-item scale was constructed to measure RSB. All variables were entered simultaneously in multivariate logistic regression models if they were significantly associated with RSB in bivariate analyses. Results: Of total, 37.7% of boys and 18.5% of girls had sexual intercourse in the past three months; of them, 69.6% of boys and 52.5% of girls were involved in commercial sex. Only 43.3% of boys and 6.5% of girls reported always using condom with unpaid regular partners in the past three months. Among sexually active girls, 43.5% reported having been pregnant and of them, 42.4% reported having induced abortion as a result of their most recent pregnancy. After adjustment, boys with higher levels of RSB were significantly more likely to live in an urban area, to have completed ≥ 9 years of formal education, and to be not currently living with parents. In contrast, girls with higher levels of RSB were significantly less likely to have completed ≥ 9 years of formal education and to have both parents alive. Both boys and girls with higher levels of RSB were significantly more likely to be in the age group of 20-24, to be not currently in school, to be employed, to becurrent alcohol drinkers, to becurrent heavy alcohol drinkers, to be current illicit drug users, and to have been tested for HIV. Conclusions: Unmarried young people in this study are exposed to several sexual reproductive health problems such as HIV and sexually transmitted infections, unwanted pregnancy, and unsafe abortion. These findings suggest the need for research and prevention programs for these key populations taking into account risk factors identified in this study.