Objective: The aim of this study was to assess whether the association between attachment representations, attitudes toward infidelity, interest in alternatives and sexual and emotional extradyadic involvement (EDI), both face-to-face and online, was mediated by the level of commitment in the relationship. Design and Method: The sample of this cross-sectional study consisted of 628 participants (201 men and 427 women), with a mean age of 23.36 years (SD = 3.91), all of whom reported being in an exclusive dating relationship for an average of 34 months. Participants completed the following self-report questionnaires: Extradyadic Behaviors Inventory, Attitudes toward Infidelity Scale, Experience of Close Relationships – Short Form, and Investment Model Scale. Results: Avoidant attachment was significantly associated with more positive attitudes toward infidelity, greater interest in alternatives, and all forms of EDI. Attachment-related anxiety was only associated with emotional EDI. The association between attachment-related avoidance and more favorable attitudes toward infidelity, greater interest in alternatives and both face-to-face and online EDI was mediated by low levels of commitment. No mediating effects were found for attachment-related anxiety. Conclusions: Avoidantly attached individuals indicated greater interest in alternatives and propensity to engage in EDI, especially when the levels of commitment in the relationship were low. These results are consistent with the literature suggesting that avoidant individuals engage in extradyadic behaviors as a way to meet their desires of independency. Commitment emerges as a relevant variable clarifying the involvement in extradyadic behaviors that may compromise the relational well-being.