Objective A family systems framework was used to examine the reciprocal influences of parents' romantic attachment security, marital adjustment, and the coparenting alliance. Background Research indicates that adult attachment strategies are predictive of adult romantic relationships, but there is less evidence linking adult romantic attachment to the ability to effectively coparent. Furthermore, much of the prior coparenting literature has focused on direct paths and has not accounted for mutual influence within parental dyads, despite an increased awareness of the interdependence among familial roles and a push to understand familywide dynamics. Method A community sample of 86 heterosexual couples with a residential child between 8 and 11 years of age completed the Experiences in Close Relationships Scale, the Dyadic Adjustment Scale, and the Coparenting Scale as part of a larger study on family processes in middle childhood. Multilevel models were conducted utilizing the actor‐partner interdependence model. Results Compared to their low attachment anxiety counterparts, spouses with higher attachment anxiety and avoidance reported lower levels of marital adjustment, less coparenting cooperation, and more coparenting conflict. Findings indicated that marital adjustment mediates the relationship between romantic attachment style and perceptions of coparenting. Conclusion Results highlight the benefit of conceptualizing parental attachment, marital, and coparental subsystems within a systemic framework and suggest that a healthy marital relationship is an important intervening factor that helps explain links between attachment security and the coparenting alliance. Implications Findings underscore the importance of evaluating and treating multiple levels of the family system and suggest that therapeutic treatment of the marital relationship may be associated with a healthier coparenting dynamic.