This study examined changes in first-time parents’ marital quality over the transition to parenthood as predictors of their coparenting quality and mothers’ and fathers’ involvement in parenting and support of their spouse’s coparenting. Mothers and fathers (N = 96 couples) individually completed measures of marital conflict and satisfaction prenatally, at 8 months, and at 24 months. Triadic family interactions observed at 24 months were coded for coparenting quality (cooperative and competitive coparenting), and coparenting dynamics (fathers’ involvement in parenting, mothers’ involvement in parenting, fathers’ support of mothers’ parenting, and mothers’ support of fathers’ parenting). Latent growth curve modeling demonstrated that declines in fathers’ marital satisfaction predicted higher competitive coparenting and lower father involvement in parenting, and increases in fathers’ marital conflict predicted lower cooperative coparenting. Increases in mothers’ marital conflict predicted mothers’ lower support of fathers’ parenting. Thus, declines in fathers’ marital quality may have more direct effects on coparenting quality, whereas declines in mothers’ marital quality may indirectly affect coparenting through mothers’ support of their spouse’s parenting.