Rates of depression have risen by more than 18 per cent since 2005, but a lack of support for the mental health combined with a common fear of stigma means many don't get the treatment they need to live healthy, productive lives.
"These new figures are a wake-up call for all countries to re-think their approaches to mental health and to treat it with the urgency it deserves," WHO Director-General Margaret Chan said.
The WHO is running a mental health campaign to tackle stigma and misconceptions. "For someone living with depression, talking to a person they trust is often the first step towards treatment and recovery," said Shekhar Saxena, director of the WHO's mental health department.
Depression is a common mental illness characterised by persistent sadness and a loss of interest and lack of ability in everyday activities and work. It affects around 322 million people worldwide.
Depression also increases the risk of several major diseases and disorders including addiction, suicidal behaviour, diabetes and heart disease, which are themselves among the world's biggest killers.
The WHO expressed concern that in many countries there is little or no support for people with mental health disorders, and said only around half of people with depression get treatment in wealthier nations.
On an average just three per cent of government health budget is spent on mental health, varying from less than one per cent in poor countries to five percent in affluent ones, according to the WHO.
"A better understanding of depression and how it can be treated ... is just the beginning," said Mr Saxena. "What needs to follow is sustained scale-up of mental health services accessible to everyone, even the most remote populations."