Coparenting between biological parents is a strong predictor of child adjustment. To date, however, little is known about the coparenting dynamics between parent and stepparent in stepfamilies. This study aimed at exploring the links between coparenting in the mother-stepfather dyad and child behavior in stepfamilies compared with the links between mother-father coparenting and child behavior in first-marriage families. Two modes of coparenting were assessed: overt coparenting, that is, coparental behaviors in the presence of the child, and covert coparenting, that is, the way each parent speaks of the other parent to the child. The sample (N = 80) comprised 48 stepfamilies and 32 first-marriage families with a child between 7 and 13 years old. Overt coparenting was assessed through direct observation in the standardized situation of the PicNic Game. Covert coparenting and child behavior were assessed through mother-reported questionnaires. Results showed (a) more covert coparenting behaviors in first-marriage families, (b) no differences in overt coparenting, (c) more child difficulties reported in stepfamilies, (d) less optimal overt coparenting being linked with more difficulties in children in both family structures, and (e) an interaction effect between family structure and coparenting, showing that overt coparenting is linked with child behavior mainly in stepfamilies.