Objective: To investigate whether maternal common mental disorders (CMD) in the postnatal period are prospectively associated with child development at 2.5 and 3.5 years in a rural low-income African setting. Methods: This study was nested within the C-MaMiE (Child outcomes in relation to Maternal Mental health in Ethiopia) population-based cohort in Butajira, Ethiopia, and conducted from 2005 to 2006. The sample comprised of 496 women who had recently given birth to living, singleton babies with recorded birth weight measurements, who were 15 to 44 years of age, and residing in six rural sub-districts. Postnatal CMD measurements were ascertained 2 months after delivery. Language, cognitive, and motor development were obtained from the child 2.5 and 3.5 years after birth using a locally adapted version of the Bayley Scales of Infant Development (3rd Ed). Maternal CMD symptoms were measured using a locally validated WHO Self-Reporting Questionnaire. A linear mixed-effects regression model was used to analyze the relationship between postnatal CMD and child development. Results: After adjusting for confounders, there was no evidence for an association between postnatal CMD and overall child development or the cognitive sub-domain in the preschool period. There was no evidence of effect modification by levels of social support, socioeconomic status, stunting, or sex of the child. Conclusions: Previous studies from predominantly urban and peri-urban settings in middle-income countries have established a relationship between maternal CMD and child development, which contrasts with the findings from this study. The risk and protective factors for child development may differ in areas characterized by high social adversity and food insecurity. More studies are needed to investigate maternal CMD’s impact on child development in low-resource and rural areas.