Few studies have examined sexual partnerships and HIV risk in diverse samples of African American/black and Hispanic/Latino adolescent and young adult men who have sex with men (YMSM), a group that have a high burden of HIV in the U.S. A community-venue recruitment approach was used, which identified significant differences in HIV risk by sexual partner type among 1215 YMSM. Those with casual partners had a higher number of sexual partners, had more STIs, and were more likely to engage in transactional sex, to use alcohol, marijuana, or other substances compared with those with main partners only. Among those with female sexual partners, many used condoms "every time" when engaging in vaginal sex with casual partners, but a sizeable proportion "never/rarely" used condoms with their main partners. Our findings demonstrate a need for tailored HIV prevention education and counseling with necessary skills regarding consistent and correct condom use with all sexual partnerships.