Two studies were conducted to identify variables associated with hypothetical infidelity forgiveness and promote forgiveness by manipulating implicit theories of relationships (ITRs; destiny/growth beliefs). Study 1 assessed the relationship between the type of behaviour, sex of the forgiver, ITRs and infidelity forgiveness. Study 2 investigated the causal relationship between ITRs and infidelity forgiveness (including attachment insecurity as a moderator). Results revealed that male participants forgave a partner's infidelity to a greater extent than female participants and that solitary behaviours were rated as most forgivable, followed by emotional/affectionate and technology/online behaviours, and sexual/explicit behaviours as least forgivable. Male participants (not female participants) induced to endorse growth beliefs forgave a partner's emotional/affectionate and solitary infidelity to a greater extent than those induced to endorse destiny beliefs; attachment insecurity moderated this relationship. These results have important implications for researchers and practitioners working with couples in distress.