Children more than the age of 12 should wear masks, in line with recommended practice for adults in their country or area, WHO said in a guidance issued by them.
It admits little is known about how children transmit the COVID-19 but cites evidence that others can be infected by teenagers in the same way as adults.
Children aged five and under should not usually wear masks, the WHO said.
Over 800,000 people have now died with coronavirus globally.
At least 23 million cases of infection have been registered, as stated by Johns Hopkins University, with most of them recorded in the US, Brazil and India.
Nevertheless, the real number of people who have had the virus is believed to be far higher, because of the insufficient testing and asymptomatic cases.
The numbers have been increasing again in countries as diverse as South Korea, EU states and Lebanon.
WHO chief Tedros Adhanom Ghebreyesus has said he hopes the pandemic will be over in two years but a top scientific adviser in the UK warned Covid-19 might never be eradicated, with people needing regular vaccinations.
