This study used data on currently married and cohabiting men aged 15 to 64 years from the 2003 Nigeria Demographic and Health Survey to examine the prevalence of and factors associated with extramarital sex. The results show that 16% engaged in extramarital sex in the 12 months preceding the survey and had an average of 1.82 partners. The results also show statistically significant association between extramarital sex and ethnicity, religion, age, age at sexual debut, education, occupation, and place of residence. Based on the study results, it could be concluded that significant proportions of Nigerians are exposed to HIV infection through extramarital sex. A fundamental behavioral change expected in the era of HIV/AIDS is the inculcation of marital fidelity and emotional bonding between marital partners. The promotion of condom use among married couples should be intensified to protect women, a large number of whom are exposed to HIV infection from their spouses who engage in unprotected extramarital sex. And, because of gender-based power imbalances within the family, a large number of the women are unable to negotiate consistent condom use by their partners.