Background: Maternal depression and unhealthy diet are well-known risk factors for adverse child emotional-behavioural outcomes, but their developmental relationships during the prenatal and postnatal periods are largely uncharted. This study sought to examine the inter-relationships between maternal depression symptoms and unhealthy diet (assessed during pregnancy and postnatal periods) in relation to child emotional-behavioural dysregulation (assessed at the ages of 2, 4 and 7 years). Method: In a large prospective birth cohort of 7814 mother-child pairs, path analysis was used to examine the independent and inter-related associations of maternal depression symptoms and unhealthy diet with child dysregulation. Results: Higher prenatal maternal depression symptoms were prospectively associated with higher unhealthy diet, both during pregnancy and the postnatal period, which, in turn, was associated with higher child dysregulation up to the age of 7 years. In addition, during pregnancy, higher maternal depression symptoms and unhealthy diet were each independently associated with higher child dysregulation up to the age of 7 years. These results were robust to other prenatal, perinatal and postnatal confounders (such as parity and birth complications, poverty, maternal education, etc.). Conclusions: Maternal depression symptoms and unhealthy diet show important developmental associations, but are also independent risk factors for abnormal child development.