The epidemic is over two-and-half years since it broke out in West Africa.
In a statement in Cape Town, South Africa, WHO remarked that the declaration on Liberia came nine days after it announced that neighbouring Guinea, where the epidemic broke out in December, 2013 was also free of it.
Meanwhile the outbreak had been declared over in Sierra Leone in mid-March.
Guinea, Liberia and Sierra Leone were the three countries most affected by the virus, which killed over 11,000 people.
Report says Liberia now enters a 90-day period of heightened surveillance to ensure that new cases are identified before they spread.
The latest announcement is the fourth time Liberia has been declared free of Ebola. The last three cases were reported in early April, with the last patient testing negative on April 28th.