The United Nations' draft Sustainable Development Goals (SDGs) only briefly mention mental health. In the context of a growing burden of disease due to mental disorders and psychosocial disabilities, the inclusion of a clear mental health target and indicators in the SDGs will acknowledge the needs and rights of hundreds of millions of people. It will mobilise international funding and policy development, and support other SDGs; it will also strengthen mental health structures, governance and services in low- and middle-income countries. We argue that for a just, sustainable and inclusive post-2015 development agenda, it is vital that the United Nations includes a clear mental health target and indicators in the SDGs.