The current study examined relations between child temperament--specifically, negative emotionality--and parents' supportive and undermining coparenting behavior, and further tested whether marital adjustment moderated relations between child negative affect and coparenting. One-hundred eleven two-parent families with a 4-year old child participated in this study. Parents completed questionnaires to provide information on children's negative affectivity, marital adjustment, and the quality of their coparenting relationships. Furthermore, parents and children participated together in two 10-minute task-oriented interactions that were coded to assess coparenting behavior. As hypothesized, parents of children higher on levels of negative affect demonstrated greater undermining coparenting behavior. In addition, marital adjustment moderated relations between children's negative affect and parents' supportive coparenting behavior. However, contrary to expectations, couples with higher levels of marital adjustment were most vulnerable to effects of child negativity on supportive coparenting. Results suggest that high-quality marital relationships may not buffer the coparenting relationship from the effects of temperamentally difficult preschoolers.