By 2015, the number of people globally living with depression, according to a revised definition, had reached 322 million, up 18.4 percent since 2005, the UN agency said on Thursday.
"These new figures are a wake-up call for all countries to rethink their approaches to mental health and to treat it with the urgency that it deserves," WHO chief Margaret Chan said in a statement.
According to the agency's definition, depression is more than just a bout of the blues.
It is a "persistent sadness and a loss of interest in activities that people normally enjoy, accompanied by an inability to carry out daily activities for two weeks or more."
Lack of energy, shifts in appetite or sleep patterns, substance abuse, anxiety, feelings of worthlessness and thoughts of self-harm or suicide are also common, and can wreak havoc on entire families.
The drop in productivity, and other medical conditions often linked to depression, also takes a financial toll, with the global cost estimated at $1 trillion annually, the WHO said.
Even in the most developed countries, around half of people suffering from depression are not diagnosed or treated, and the percentage soars to between 80 and 90 percent in less developed nations.
According to the WHO, every dollar invested in improving access to treatment leads to a return of $4 in better health and productivity.
About 800,000 people commit suicide worldwide every year, amounting to one suicide every four seconds.