OBJECTIVES: (1) To investigate the impact of perinatal common mental disorders (CMD) in Ethiopia on the risk of key illnesses of early infancy: diarrhoea, fever and acute respiratory illnesses (ARI) and (2) to explore the potential mediating role of maternal health behaviours. DESIGN: Population-based cohort study. SETTING: Demographic surveillance site in a predominantly rural area of Ethiopia. PARTICIPANTS: 1065 women (86.3% of eligible) in the third trimester of pregnancy were recruited and 954 (98.6%) of surviving, singleton mother-infant pairs were followed up until 2 months after birth. MAIN EXPOSURE MEASURE: High levels of CMD symptoms, as measured by the locally validated Self-Reporting Questionnaire (SRQ-20 ≥6), in pregnancy only, postnatally only and at both time-points ('persistent'). MAIN OUTCOME MEASURES: Maternal report of infant illness episodes in first 2 months of life. RESULTS: The percentages of infants reported to have experienced diarrhoea, ARI and fever were 26.0%, 25.0% and 35.1%, respectively. Persistent perinatal CMD symptoms were associated with 2.15 times (95% CI 1.39 to 3.34) increased risk of infant diarrhoea in a fully adjusted model. The strength of association was not affected by including potential mediators: breast feeding practices, hygiene, the infant's vaccination status or impaired maternal functioning. Persistent perinatal CMD was not associated with infant ARI or fever after adjusting for confounders. CONCLUSIONS: Persistent perinatal CMD was associated with infant diarrhoea in this low-income country setting. The observed relationship was independent of maternal health-promoting practices. Future research should further explore the mechanisms underlying the observed association to inform intervention strategies.