Postpartum depression leaves irredeemable impacts on a mother's mental health and her child. Little is known about the relationship between immature defense mechanisms and body image concerns or postpartum depression. The present study examines the mediating role of body image concerns in the relationship between immature defense mechanisms and postpartum depression. In a correlational study, 227 women were selected through a convenience sampling method from all women in the postpartum period in medical centers in 2021 in Tehran, Iran. The participants were asked to fill out the Edinburgh Postnatal Depression Scale (EPDS), the Defense Style Questionnaire-40 (DSQ-40) by Andrews et al., and the Body Image Concern Inventory (BICI). The collected data was analyzed using the IBM SPSS Amos 24 and SPSS 21 software. There was a positive correlation between body image concern, immature defense mechanisms, and postpartum depression. Furthermore, body image concern was a mediator in the relationship between immature defense mechanisms and postpartum depression. The immature defense mechanisms had an indirect positive impact on depression due to the concerns over one's body image after giving birth (P > 0.05). Immature defense mechanisms and body image concerns can increase the risk of postpartum depression. Therefore, providing mental health and psychiatric services to expecting mothers is an effective tool to decrease the activation of immature defense mechanisms, which would reduce their body image concerns and prevent them from falling into postpartum depression.