The transition to first-time parenthood can be challenging for couples. Using a sample of 848 ethnically diverse couples from the Fragile Families and Child Wellbeing Study across the first 3 years of parenthood, we investigated the longitudinal and dyadic associations of each parents’ parental stress, supportive coparenting, and relationship quality. Results from an actor–partner interdependence model indicated that supportive coparenting significantly predicted higher relationship quality for both mothers and fathers. Fathers’ supportive coparenting significantly buffered the effects of mothers’ parental stress on relationship quality. Also, the unique dyadic contexts of each parents’ supportive coparenting, and also both partners’ parental stress were significantly associated with relationship quality. Clinical implications from these findings are discussed through commonly used clinical theories.