The vaccine is the first to prevent infection from one of the most lethal known pathogens, according to the results published in The Lancet journal.
"While these compelling results come too late for those who lost their lives during West Africa's Ebola epidemic, they show that when the next Ebola outbreak hits, we will not be defenceless," said Marie-Paule Kieny, WHO's Assistant Director-General for Health Systems and Innovation, and the study's lead author.
In the most recent Ebola outbreak in West Africa that started in late 2013, more than 11,000 people lost their lives. The WHO removed the global emergency tag for the disease early this year.
The vaccine, called rVSV-ZEBOV, was studied in a trial involving 11,841 people in Guinea during 2015.
Among the 5,837 people who received the vaccine, no Ebola cases were recorded 10 days or more after vaccination.
In comparison, there were 23 cases 10 days or more after vaccination among those who did not receive the vaccine.
The trial was led by the World Health Organization, together with Guinea's Ministry of Health and other international partners.
The vaccine's manufacturer, Merck, Sharpe & Dohme, this year received Breakthrough Therapy Designation from the United States Food and Drug Administration and PRIME status from the European Medicines Agency, enabling faster regulatory review of the vaccine once it is submitted.
The vaccine works by replacing a gene from a harmless virus known as vesicular stomatitis virus (VSV) with a gene encoding an Ebola virus surface protein. The vaccine does not contain any live Ebola virus.
Earlier trials had shown the vaccine to be protective in animals, and be safe and produce an immune response in humans.
Since Ebola virus was first identified in 1976, sporadic outbreaks have been reported in Africa.
But the 2013-2016 West African Ebola outbreak, which resulted in more than 11,000 deaths, highlighted the need for a vaccine.
The trial took place in the coastal region of Basse-Guinee, the area of Guinea still experiencing new Ebola cases when the trial started in 2015.
To assess safety, people who received the vaccine were observed for 30 minutes after vaccination, and at repeated home visits up to 12 weeks later.
Approximately half reported mild symptoms soon after vaccination, including headache, fatigue and muscle pain but recovered within days without long-term effects.
Two serious adverse events were judged to be related to vaccination (a febrile reaction and one anaphylaxis) and one was judged to be possibly related (influenza-like illness).
All three recovered without any long term effects, the study reported.