Objective To assess the possible relationship between serum levels of 25[OH]D (25-hydroxyvitamin D) collected 24 hours after delivery and postpartum depression in a Chinese cohort sample.Design Cohort study.Setting One city hospital in Beijing, China.Population Women delivering a full-term, singleton, live-born infant at one city hospital in Beijing between August 2013 and November 2013.Methods Women were enrolled immediately postpartum. A blood sample was obtained 24-48 hours after childbirth to test serum levels of 25[OH]D. Participation consisted of a visit to an obstetric unit 3 months after delivery.Main outcome measure At 3 months' postpartum, women were screened for depression using the Edinburgh Postnatal Depression Scale (EPDS). The primary outcome measure was a prespecified EPDS score of ≥12.Results During the study period, 323 women were admitted. In all, 248 agreed to enrol and 213 completed 3 months' follow-up (21 were lost to follow-up and 14 withdrew). Of the 213 women who were included, 26 (12.2%) were considered to meet criteria for postpartum depression. Serum 25[OH]D levels in women with no postpartum depression were significantly higher than those in women with postpartum depression (P < 0.0001). Based on the receiver operating characteristic curve, the optimal cutoff value for serum 25[OH]D level as an indicator for screening for postpartum depression was estimated to be 10.2 ng/ml, with an area under the curve of 0.801 (95%CI 0.704-0.896). In multivariate analysis, there was an increased risk of postpartum depression associated with 25[OH]D levels ≤10.2 ng/ml (OR 7.17,; P < 0.0001) after adjusting for possible confounders.Conclusion Our study demonstrated that lower serum 25[OH]D levels were associated with postpartum depression. This association was independent of other possible variables.