Previous research has reported that transgender women are likely to be exposed to HIV through unprotected sex with a male primary partner. We examined prevalence and correlates of unprotected sex with a primary male partner in a sample of n = 174 transgender women. Participants completed surveys on demographic characteristics, relationship dynamics with their male primary partner, sexual behavior, substance use, and psychosocial factors. Overall, 41% reported HIV positive status, 13% had another sexually transmitted infection during the past year, and 34% had unprotected sex with a male primary partner during the past 3 months. Factors associated with unprotected sex with a primary partner included living with the partner, drug use, alcohol use, education level, low self-efficacy to use condoms, and perceived discrimination. Notably, 35% of transgender women in HIV-discordant primary partnerships had unprotected sex with their male primary partner during the past 3 months, and 18% of transgender women in HIV-positive concordant primary partnerships had unprotected sex with an outside partner during the past 3 months. HIV prevention interventions for transgender women must address risk behavior in the context of primary partnerships as well as sex with concurrent partners outside the relationship. Couples-focused interventions involving transgender women and their male primary partners can be particularly promising.