Background Symptoms of depression and anxiety are common during pregnancy and the postnatal period. A risk factor for mood disorders is poor sleep quality. In this study we investigate the effects of poor subjective prenatal sleep quality on postnatal depressive and anxiety symptoms, independent of prenatal depression or anxiety, amongst pregnant women in the general population. Methods We analysed data from a subset of women taking part in a prospective cohort study, Growing Up in Singapore towards Healthy Outcomes. The participants completed the Edinburgh Postnatal Depression Scale and State-Trait Anxiety Inventory between 26 and 28 weeks of pregnancy (Time 1) and at 3 months postpartum (Time 2), and the Pittsburgh Sleep Quality Index at Time 1. Logistic regression analyses were used to investigate the associations between subjective prenatal sleep quality and postnatal depressive and anxiety symptoms, while adjusting for prenatal depressive/anxiety symptoms and education. Results Although borderline-high depressive/anxiety symptoms were the strongest predictors of postnatal depressive/anxiety, independent of this, poor subjective sleep quality during pregnancy was also associated with borderline-high postnatal depressive symptoms, but not with postnatal anxiety. Limitations Sleep quality and prenatal/postnatal mood were derived from self-reported questionnaires, which may be more susceptible to bias. Conclusion Although treatment of symptoms of prenatal depression and anxiety will be the most important for reducing postnatal depression and anxiety, in addition to that, future studies may explore treatments improving prenatal sleep quality, particularly for women with antenatal depressive symptoms.