After experiencing a relational transgression, individuals may not forgive their partner. However, unforgiveness may prove detrimental to relationship functioning for both partners, and even more so when combined with individual and relational risk factors. This study examined the associations between unforgiveness dimensions (cognitive‐evaluative, emotional‐ruminative, and offender reconstrual) and relationship under‐commitment in couples who experienced a relational transgression, and the moderating roles of attachment insecurities (attachment anxiety and avoidance) and the sample type (community vs. clinical) in these associations. The sample included 114 couples (42 from the community and 72 seeking relationship therapy); both partners completed online questionnaires. Path analyses revealed associations between the cognitive‐evaluative and offender reconstrual dimensions, and higher under‐commitment in offended partners. The association between offender reconstrual and under‐commitment was only present when offended partners exhibited low to moderate levels of attachment anxiety. The emotional‐ruminative dimension was associated with under‐commitment in both partners, but only when offended partners reported low levels of attachment avoidance. No moderation effect was found for the sample type. This study enhances understanding of post‐transgression unforgiveness and unravels some individual characteristics that are likely to affect how it relates to both partners' under‐commitment.