Objective: Many psychiatric diseases have a strong familial aggregation, but it is unknown whether postpartum depression (PPD) without prior psychiatric history aggregates in families. Methods: Based on Danish national registers, we constructed a cohort with information on 848,544 singleton deliveries (1996–2017). Women with an episode of PPD were defined as having used antidepressant medication and/or had a hospital contact for depression within 6 months after delivery. Those with psychiatric history prior to the delivery were excluded. We estimated relative risk (RR) of PPD, comparing women with female relatives with and without PPD history, respectively. Results: Overall, women with a PPD history in female blood relatives had themselves a higher risk of PPD (RR = 1.64, 95% CI 1.16–2.34). Having the first-degree female relative with PPD history was associated with a more than 2.5 times (RR = 2.65, 95% CI 1.79–3.91) increased risk of PPD. However, having the second/third-degree female relative and/or a female non-blood relative with PPD history did not increase the woman's own risk of PPD (RR = 0.58, 95% CI 0.26–1.28, RR = 1.09, 95% CI 0.83–1.44). Conclusion: Postpartum depression aggregates in families with no other psychiatric history, but the findings do not support a strong genetic trait as a major cause. Other possible mechanisms are shared environment and/or health-seeking behavior in close relationships.