Natural disasters can have serious mental health consequences. We aimed to examine the long-term effects of the 2011 Great East Japan Earthquake (GEJE) on postpartum depressive symptoms (PDS). The Tohoku Medical Megabank Project Birth and Three-Generation Cohort Study recruited pregnant women in Miyagi Prefecture from 2013 to 2016. Data from 11,403 participants were used in this study. Women were asked about their traumatic experiences of the GEJE with questions addressing threat, witness, and loss. PDS were defined as Edinburgh Postnatal Depression Scale score ≥9 at 1 month after delivery. Multiple logistic regression analyses were conducted to examine the associations of different traumatic experiences of the GEJE and number of traumatic experiences with PDS, after adjustment for age, parity, feelings toward pregnancy, education, income, social isolation, house damage caused by the GEJE, and survey year. About two-fifths of women had at least one traumatic experience of the GEJE. The prevalence of PDS at 1 month after delivery was 13.3 %. Life-threatening experience and witnessing another person's actual or threatened death were associated with PDS: the odds ratios (ORs) were 1.40 (95 % confidence interval CI, 1.24–1.59) and 1.28 (95 % CI, 1.08–1.53), respectively. Loss of close person was not associated with PDS: the OR was 1.13 (95 % CI, 0.99–1.30). Larger number of traumatic experiences of the GEJE was associated with increased risk of PDS (p for trend <0.001). PDS was self-reported. Traumatic experiences of the GEJE prior to pregnancy were associated with increased risks of PDS. •Life-threatening experience was associated with postpartum depressive symptoms.•Loss of close person was not associated with postpartum depressive symptoms.•Larger number of experiences was associated with postpartum depressive symptoms.•The long-term effect of a natural disaster on women's mental health was suggested.