Objective Cancer poses unique challenges for the couple relationship. From a relational perspective, successful adaptation may be less dependent on the circumstances of being the “patient” or the “caregiver” than on how couples successfully integrate cancer into their relationship. In this article, we study posttraumatic growth through attachment theory, which provides a useful framework to explore the role of intimate processes dyadically. Method The sample comprised 84 heterosexual married and cohabitating couples. Women were in adjuvant treatment for breast cancer or in the posttreatment phase. Both members of the couple completed measures of attachment and posttraumatic growth. Path models were used to examine associations between the constructs through the application of the actor‐partner interdependent model. Moreover, time since diagnosis was examined within the actor‐partner interdependent model as a possible moderator affecting the attachment‐posttraumatic growth associations. Results Partners' attachment security was an important predictor of individual posttraumatic growth for both members of the couple, while individuals' own attachment security was not associated with their posttraumatic growth for either member of the couple. The partner's effects were equal across gender. Additionally, time since breast cancer diagnosis did not affect the pattern of results. Conclusion Findings support the assessment of the couple dyadically and corroborate an attachment approach of the couple as an interdependent unit. Implications of the study for interventions assisting couples in oncological setting are discussed.