Child attachment undergoes major changes during middle childhood. Maternal reflective functioning (RF) is hypothesized to be an important correlate of a child’s attachment security during this period; however, the child’s gender role in this association has not been examined yet. In the present study, we used 64 mother-child (6–11 years old) dyads from a community sample to analyze the association between maternal RF and child attachment security and whether this link is moderated by gender. Maternal RF was assessed on the Parent Development Interview Revised (PDI-R2) and child attachment classifications were examined by the Child Attachment Interview (CAI). Results revealed the positive and statistically significant association between maternal RF and child attachment security in the whole sample. Further evaluations of this link in the groups of girls and boys separately indicated its significance for girls only. Finally, moderation analysis demonstrated the relation between child attachment security and maternal RF to be moderated by gender. These findings provide a new knowledge on gender role in attachment security in relation to maternal RF as well as suggest possible differentiation in the correlates on the pathway of secure attachment between girls and boys during middle childhood.