This study examined the prevalence and the determinants of risky sexual behavior (defined as having multiple sex partners and paying for sex) among male rural-to-urban migrants in China. An anonymous questionnaire was used to collect information on socio-demographics, knowledge, attitudes, and behavior associated with increased risk of risky sexual behavior from 4,069 subjects. In total 1,132 (27.8%) participants reported two or more sex partners and 802 (19.7%) participants paid for sex. A considerable proportion (29.6%-41.5%) did not use a condom during risky sexual behavior. Logistic regression analysis revealed that unmarried status (OR: 0.62, CI: 0.42-0.85 for married), earlier age at first sexual experience (OR: 0.67, 95% CI: 0.31-0.91 for ≥ 22 years old), poor perception of risk of acquiring HIV/AIDS (OR: 1.51, 95% CI: 1.33-1.96 for unlikely; OR: 2.38, 95% CI: 1.61-3.70 for impossible), frequent exposure to pornography (OR: 0.67, 95% CI: 0.60-0.81 for sometimes; OR: 0.31, 95% CI: 0.11-0.43 for never), attitudes toward legalization of commercial sex (OR: 0.39, 95% CI: 0.21-0.59 for no), peer influence (OR: 0.51, 95% CI: 0.27-0.88 for no), and not knowing someone who had/had died from HIV/AIDS (OR: 0.35, 95% CI: 0.20-0.53 for yes) were all significantly associated with having multiple sex partners. Those who paid for sex showed similar findings.