The EU and WHO also reported Tuesday that one in seven people with HIV in Europe is unaware of their infection
The UN health agency said that 40 percent of people with the virus that causes AIDS, or more than 14 million people worldwide, are unaware of their status, according to 2015 estimates.
That marks a huge improvement over just a decade earlier, when only 12 percent of people with HIV were estimated to know they had the virus.
But the continued lack of diagnosis, remains a major obstacle to implementing WHO's recommendation for everyone with HIV to be offered anti-retroviral therapy, or ART.
Today, more than 80 percent of everyone diagnosed with HIV is receiving ART.
But WHO chief Margaret Chan warned that since so many people do not know their status, "millions of people with HIV are still missing out on life-saving treatment, which can also prevent HIV transmission to others."
"HIV self-testing should open the door for many more people to know their HIV status and find out how to get treatment and access prevention services," she said.
HIV self-testing means that people can, in the privacy of their own homes, use oral fluid or blood from a finger prick to determine their status in a matter of minutes.
WHO urges anyone who tests positive to seek confirmatory tests at a health clinic, where they can receive information about the disease and how to get counselling, as well as rapid referral to prevention, treatment and care services.
Self-testing has been shown to nearly double the frequency of HIV testing among men who have sex with men, and recent studies in Kenya found that the male partners of pregnant women were twice as likely to get tested if they were offered self-testing, WHO said.
"By offering HIV self-testing, we can empower people to find out their own HIV status and also to notify their partners and encourage them to get tested as well," Gottfried Hirnschall, head of WHO's HIV department, said in the statement.
Twenty-three countries currently have national policies in place supporting HIV self-testing, while others are developing such policies, but WHO warned that in much of the world wide-scale access to the tests remains limited.
The EU and WHO also reported Tuesday that one in seven people with HIV in Europe is unaware of their infection, as 2015 marked another record year for new HIV cases in the region.
Europe registered 153,407 new cases, up from 142,000 in 2014, the WHO said, a jump driven by cases in Russia and immigrants who acquired the virus after arrival.
The WHO regional office for Europe compiles data from 53 countries.