Extradyadic sex is prevalent in adult romantic relationships and it can have numerous negative consequences. It can negatively affect relationship functioning and makes both members of a romantic dyad more susceptible to particular health risks. The present research provides evidence for an association between problem drinking and extradyadic sex. In two studies, participants completed measures of problem drinking, extradyadic sex, relationship satisfaction, and social desirability. In Study 1 (N = 353), problem drinking predicted higher rates of extradyadic sex controlling for sex, age, relationship satisfaction, and social desirability. In Study 2, this finding replicated both cross-sectionally (N = 500) and prospectively (N = 384), controlling for Time 1 extradyadic sex. Moreover, a majority of participants who engaged in extradyadic sex did not disclose this information to partners and a significant minority did not use a condom during the extradyadic incident.