Summary Depression in adolescence is associated with a range of negative outcomes and substantial risk for morbidity and mortality across the lifespan. Dietary improvement and supplementation may offer an inexpensive and acceptable adjunct to standard treatment; yet this has, to date, been largely overlooked, owing to lack of evidence and knowledge. This is important, as improving understanding of the role of diet in mental health and promotion of appropriate dietary practices could significantly reduce the personal and social impact of depression in young people. This article sets out to review the existing research literature on associations between diet and mental health in adolescence, in particular, the role of diet in the prevention and management of adolescent depression. A closer examination of associations between obesity and depression is also included, as these two conditions often co-occur. Research into diet and mental health tends to fall into two main categories: population-based observational studies and intervention studies; however, there is very little research evidence specific to adolescents. Potential implications for mental healthcare policy and practice are discussed.