The couple and coparenting relationship are theorized to influence each other in a reciprocal manner over time. Empirical evidence demonstrates cross-sectional associations between the 2 as well as prospective predictions of coparenting by relationship quality and vice versa. However, less is known about the longitudinal reciprocity between the couple relationship and coparenting from the perspective of both parents. The current study sought to examine longitudinal associations between relationship quality and coparenting support/undermining across the transition to parenthood from a dyadic perspective. Participants were 164 cohabiting heterosexual couples expecting their 1st child, assessed during pregnancy and at 6 and 36 months after birth. Actor partner interdependence modeling was used to examine, for both men and women, (a) stability over time in relationship quality and coparenting, (b) reciprocal associations between relationship quality and coparenting support/undermining, and (c) the gender differences in those associations. Moderate rank-order stability in relationship quality and coparenting support/undermining across the 1st 3 years of parenthood was demonstrated. For women, but not men, findings suggested longitudinal reciprocal associations between relationship quality and coparenting support/undermining. Specifically, our findings suggested that prenatal relationship quality sets the stage for coparenting functioning after birth for both men and women but that coparenting functioning is then connected to subsequent feelings about the romantic relationship for only women.