DAKAR, Senegal—Guinea has been declared free from transmission of Ebola, the World Health Organization said Tuesday, marking a milestone for the West African country where the original Ebola chain of transmission began two years ago.

The U.N. organization is holding a ceremony Tuesday in Conakry, the capital, to mark the step forward. More than 2,500 people have died in Guinea from Ebola. The deadly virus has killed more than 11,300 people worldwide, mostly in Liberia, Sierra Leone and Guinea.

Guinea...