This study examined associations between supportive coparenting and infant-mother and infant-father attachment security. Observed and parent-reported coparenting, and observed maternal and paternal sensitivity were assessed in a sample of 68 families with 3.5-month-old infants. Infant-mother and infant-father attachment security were assessed in the Strange Situation Procedure at 12 and 13 months of age, respectively. Observed and reported supportive coparenting were associated with greater attachment security in the infant-father, but not the infant-mother, attachment relationship. The link between observed coparenting and infant-father attachment remained after accounting for paternal sensitivity. Furthermore, child gender moderated some associations between coparenting and infant-parent attachment security. Amongst families with boys, observed and reported supportive coparenting were related to greater infant-mother and infant-father attachment security, respectively. Coparenting was unrelated to infant-mother or infant-father attachment security amongst families with girls. Results highlight a possible link between the coparental and father-child relationships and the need to consider both parent and child gender when examining associations between family functioning and attachment. (Contains 3 tables and 2 figures.)