Drawing on a two-wave, multimethod, multi-informant design, this study provides the first test of a process model of spillover specifying why and how disruptions in the coparenting relationship influence the parent–adolescent attachment relationship. One hundred ninety-four families with an adolescent aged 12–14 (M age = 12.4) were followed for 1 year. Mothers and adolescents participated in two experimental tasks designed to elicit behavioral expressions of parent and adolescent functioning within the attachment relationship. Using a novel observational approach, maternal safe haven, secure base, and harshness (i.e., hostility and control) were compared as potential unique mediators of the association between conflict in the coparenting relationship and adolescent problems. Path models indicated that, although coparenting conflicts were broadly associated with maternal parenting difficulties, only secure base explained the link to adolescent adjustment. Adding further specificity to the process model, maternal secure base support was uniquely associated with adolescent adjustment through deficits in adolescents' secure exploration. Results support the hypothesis that coparenting disagreements undermine adolescent adjustment in multiple domains specifically by disrupting mothers' ability to provide a caregiving environment that supports adolescent exploration during a developmental period in which developing autonomy is a crucial stage-salient task.