Relationship quality often declines following the birth of child, likely reflecting in part the shift towards role traditionalization that occurs through gender specialization. The current study used longitudinal data from the Fragile Families and Child Well-Being Study, an urban birth cohort in 2000 consisting of structured interviews of mothers and fathers who were followed over 5 years (n = 1275), to examine whether low levels of fathers' involvement and coparenting, two indicators of role traditionalization, were linked to negative trajectories of mothers' and fathers' relationship quality for couples whose first child was born in marriage or cohabitation. We carefully consider union transitions in the 5 years postpartum by including between-subjects variables indicating that the parents were continually married, continually cohabiting, were cohabiting at the child's birth and got married after, or were cohabiting or married at the child's birth but subsequently separated. As anticipated, both fathers' involvement and coparenting were positively associated with parents' reports of relationship quality, more so for mothers than for fathers and especially for cohabiting mothers, buffering the decline in mothers' and fathers' relationship quality that typically accompanies the birth of a child. These findings underscore the importance of the father role, not only for the well-being of the child (as we know from other research) but also for the relationship of the parents. Fathers should be encouraged and supported to take an active role in parenting through educational programs and public policy (e.g., paid paternity leave).