Background/objectives <p>The effect of the frequency of nighttime snacking during pregnancy on postpartum depression (PPD) remains unclear. This prospective cohort study aimed to explore the association between the frequency of nighttime snacking during pregnancy and the incidence of PPD.</p> Methods <p>This study included 609 community-dwelling pregnant women who were recruited between July 2019 and July 2022. Data on the frequency of nighttime snacking and sociodemographic, economic, and lifestyle factors were obtained using self-administered questionnaires. Data on dietary intake were obtained using a validated brief self-administered diet history questionnaire. PPD was assessed using the Japanese version of the Edinburgh Postnatal Depression Scale (EPDS). The incidence of PPD (EPDS score of ≥9) based on the frequency of nighttime snacking was analyzed using multiple logistic regression analysis.</p> Results <p>After adjusting for potential confounding factors, the odds ratio (OR) of PPD in participants who had a frequency of nighttime snacking 3 times/week or more compared with those who had a frequency of nighttime snacking less than 1 time/week was 2.59 (95% confidence interval, 1.14–5.86). Sensitivity analysis was repeated for the excluded participants with depressive symptoms during pregnancy, yielding similar results. The OR of PPD in those who indulged in nighttime snacking ≥3 times/week was the highest (2.81; 95% confidence interval, 1.13–6.96).</p> Conclusions <p>The present study clarified the association between the frequency of nighttime snacking and PPD. Further studies with large and diverse samples are needed to overcome any potential selection bias.</p>