INTRODUCTION: Mother's bond to the infant in the postpartum period plays an important role in the subsequent mother-infant relationship and the infant's socio-emotional functioning. Several maternal characteristics, such as attachment style and psychological flexibility, may contribute to the quality of mother-infant bonding, though literature examining these variables is still scarce. The present study aimed to examine the impact of mother's attachment on mother-infant bonding in the first month postpartum and the mediating role of psychological flexibility on that association. METHODS: Participants were 226 mothers of an infant up to 9 months old, who reported on their own attachment style (in terms of anxiety, comfort with proximity, trust in others), psychological flexibility (in terms of openness to experience, behavioural awareness, valued action) and mother-infant bonding. RESULTS: Results showed that mother's attachment anxiety predicted a bond with the infant directly and indirectly via mother's psychological flexibility, specifically through behavioural awareness and valued action. Trust in others had an impact on mother-infant bonding through behavioural awareness, whereas comfort with proximity influenced mother-infant bond indirectly, via valued action. Finally, mothers' civil status, schooling and number of children were relevant to better understand the variance of our mediating and dependent variables. DISCUSSION: Our findings highlight the importance of mother's attachment and psychological flexibility in promoting the quality of mother-infant bonding, which can inform future intervention programmes targeting modifiable factors, such as psychological flexibility, to promote early positive parent-infant relationships, particularly for single, first-time mothers, with higher levels of education.