The purposes of the present research were to examine the relationship between attachment and extradyadic sex and to investigate a mediator of this relationship. Study 1 showed that attachment avoidance was positively associated with extradyadic sex, while attachment anxiety was unrelated to it. These results were maintained after controlling for sexual satisfaction, sexual desire, gender, and age. Study 2 replicated the results from Study 1, while also controlling for couple adjustment. Study 3 used a prospective design and further showed that concerns with the partner's desire for engagement mediated the relationship between attachment avoidance and extradyadic sex. Overall, the findings suggest that attachment avoidance increases people's irritation relative to their partner's desire for engagement which, in turn, increases their likelihood to engage in extradyadic sex. The possibility that individuals characterized by attachment avoidance might use extradyadic sex as a way to distance themselves from their partner is discussed.