Introduction: Postpartum Depression (PPD) is a serious mental condition that may affect up to 20% of women. Several factors can contribute to this problem, including hormonal changes, stress, and a history of depression. One of the factors that has recently received attention is weight gain during pregnancy. The purpose of this study is to investigate the relationship between weight gain during pregnancy and postpartum depression. Methods and Materials: In this cross-sectional study, 300 pregnant women who participated in the health centers of Yazd were examined. The data included demographic information, weight gain during pregnancy, and postpartum depression. Weight gain was classified according to the Institute of Medicine (IOM) guidelines, and postpartum depression was assessed using the Edinburgh Postpartum Depression Scale (EPDS). Data were analyzed using statistical analysis independent T-tests and Pearson's correlation coefficient. Results: The results showed that women who gained more than the recommended weight during pregnancy (more than 16 kg) were more likely to develop postpartum depression than women who gained weight within the recommended range. The average EPDS score was 14.2 in women with excessive weight gain and 10.5 in women with weight gain within the recommended range. The difference between the two groups was statistically significant (p 0.01). Conclusion and Discussion: Weight gain more than recommended during pregnancy can be a risk factor for postpartum depression. These findings emphasize the need for intervention programs for weight management during pregnancy and psychological support after delivery.