Growing evidence from preclinical studies, epidemiology, and randomized controlled trials supports a causal role for diet quality in mental disorder risk, and clinical psychiatric guidelines now place diet, along with other life-style behaviors, as foundational treatment targets for mood disorders. Diet quality in the perinatal period is related to both mothers’ mental health and children’s emotional and neurodevelopment outcomes. The human gut microbiota composition is influenced by diet, and emerging evidence suggests that disturbances in gut microbiota, at least in part, mediate these relationships. Thus, optimizing maternal diet should be prioritized as part of a multidisciplinary approach for promoting physical and mental health in mothers and their offspring. This paper addresses the current evidence base and discusses its application in perinatal health care.