Nigerian men who have sex with men (MSM) have a high burden of HIV infection and are known to engage in bisexual behavior. This study presents the first data on characteristics and correlates of Nigerian men having sex with men and women (MSMW) in three Nigerian cities. Five hundred and fifty-seven MSM who engaged in anal sex with men completed a behavioral survey; 48.1% of these MSM also engaged in sex with women in the previous 2 months. MSMW displayed high levels of risky sexual behavior with female sex partners; casual (56.0%) and multiple female partners were common (69.0%) and 66.0% had unprotected vaginal sex. As much as 45.1% MSMW had anal sex with female partners of which 74.0% did not use protection in the 2 months prior. In bivariate analyses, bisexual behavior was associated (p<0.05) with being married or living with a women (OR 5.0, 95% CI = 2.6-9.4), less education (OR 2.0, 95% CI = 1.4-3.0), bisexual/straight identity (OR 2.3, 95% CI = 1.6-3.2), being an insertive partner (OR 3.0, 95% CI = 1.9-4.5), being HIV-negative (OR 1.6, 95% CI = 1.1-2.5), living in Lagos (OR 2.3, 95% CI = 1.7-2.2), being Muslim (OR 1.7, 95% CI = 1.1-2.5), and being away from home (OR 1.5, 95% CI = 1.0-2.1). In the multivariate model, being married to or living with a woman (AOR = 5.1; 95% CI = 2.5-10.3), bisexual/straight identity (AOR = 2.2; 95% CIs = 1.5-3.3), being an insertive partner (AOR = 3.0; 95% CI = 1.9-4.9), being away from home (AOR = 1.6; 95% CI = 1.1-2.3) and living in Lagos (AOR = 1.7; 95% CI = 1.0-2.8) remained significant (p< 0.05). High levels of bisexual behavior exist among Nigerian MSM, and these men engage in risky sexual behaviors with both male and female sex partners. While decriminalization of same-sex behavior in Nigeria will promote access to HIV prevention programs, current MSM interventions must incorporate information on safe sex with both male and female sex partners.