The advice applies even if a person has no symptoms.
It comes a few weeks after doctors discovered the virus in the sperm of an Italian man six months after he first had Zika symptoms.
Zika is spread in bodily fluids.
The main risk of catching the disease is from infected mosquitoes via bites.
Previously, WHO had said men without symptoms only needed to use condoms or abstain from sex for eight weeks as a precaution against spreading Zika.
According to experts, once a person has been infected, he or she is likely to be protected from future infections.
Many people infected with Zika won't have symptoms or will only have mild ones - a fever, rash and muscle/joint aches.
Zika in pregnancy is the major concern because the virus can damage the unborn child.
Brazil has been the hardest-hit country in terms of Zika infections and there were some concerns about hosting the Olympics there for this reason.
There have been no reports of confirmed cases of Zika virus among people who attended the Games, both during and since their return.
According to the WHO, 11 countries have reported cases of sexually-transmitted Zika.
More than 60 countries and territories have continuing Zika transmission from local mosquitoes.
Public Health England said it was reviewing the WHO's new advice and would change its own guidance if necessary. It said the risk to people in the UK remained very low.