Family boundary ambiguity refers to a lack of clarity as to who is in and who is out of the family system. Few studies have examined this concept in the stepfamily context, which is problematic because such definitional problems hinder our understanding of close relationships in stepfamilies. Based on a nationally representative sample of first-married, remarried, and cohabiting couples with minor children, this study investigates the prevalence and nature of boundary ambiguity in stepfamilies (families that include children from previous unions) and the relationship between boundary ambiguity and couples' relationship quality and stability. Results suggest that boundary ambiguity is much more prevalent in stepfamilies than original two-parent families and that boundary ambiguity is associated with the structure of the stepfamily. Boundary ambiguity is negatively associated with the quality of the couples relationship and stability of the union, but only from the perspective of wives and female partners.