Using couple data from a national survey, this article examines couple's characteristics associated with extramarital sex among Nigerian men. We found 15.4% of married men had extramarital sex in the past 12 months. Extramarital sex was significantly associated with men's attitude toward extramarital sex (OR = 1.7 [1.4–3.0]), early sexual debut (OR = 1.9 [1.6–2.3]), alcohol use (OR = 1.7 [1.4–2.1]), and intimate partner violence against wife (IPV) (OR = 1.4 [1.2–1.7]). Increased wife's education was associated with decreased husband's extramarital sex. Men living in rural areas and in the Central and Southern regions were also more likely to have extramarital sex. The findings suggest useful implications for HIV prevention programs in Nigeria. Interventions should focus on influencing social norms around protective behaviors for men to avoid risks associated with extramarital sex and IPV, helping men to change attitudes toward extramarital sex and IPV, and promoting delay in age at first sex among young men.