The health body had previously said there was no evidence face masks worked but now agrees they are better than nothing.
WHO director-general Tedros Adhanom Ghebreyesus announced the change in recommendation on Friday, saying the organisation is now recommending people wear masks in areas where there is widespread COVID-19 transmission.
He said people should wear them in places such as public transport or shops, where social distancing is not possible.
People over the age of 60 or those with underlying medical conditions should specifically wear a medical mask where social distancing cannot be maintained, he added.
Countries around the world have adopted the use of face masks, despite WHO previously saying they were not effective.
Britons using public transport must wear face coverings from 15 June after Boris Johnson changed his mind about their effectiveness.
Previously, the WHO only recommended healthcare workers, those with COVID-19 and their care givers wear masks.
Mr Tedros said the widespread use of face masks is still not yet supported by high quality or direct scientific evidence.
However, he said a growing amount of observational evidence from several countries who are recommending the use of face masks, plus the difficulty of social distancing in many settings, is enough for WHO to change its advice.
He emphasised that masks on their own will not protect people from getting COVID-19 and hand washing, social distancing and other measures are still important.
The director-general added that health workers in areas with widespread transmission should now wear medical masks in all areas of health facilities, not just where confirmed COVID-19 patients are.
Doctors working in cardiology or others wards, for example, should continue to wear a medical mask even if there are no known coronavirus patients, he said.
The UK government has also said all staff in hospitals in England will be expected to wear surgical masks from 15 June, and all visitors and outpatients will have to wear face coverings at all times in hospital.
Evidence from the Scientific Advisory Group for Emergencies (SAGE) previously confirmed face coverings can help reduce the risk of transmission if you are suffering from coronavirus, but not showing symptoms.
NHS staff already wear face masks in clinical areas within two metres of a patient, but this new guidance applies to everyone working in all areas of a hospital.