WHO representative Rui Gama Vaz, speaking in the capital Abuja, said it was a "spectacular success story".
Nigeria won praise for its swift response after a Liberian diplomat brought the disease there in July.
The outbreak has killed more than 4,500 people in West Africa, mostly in Liberia, Guinea, and Sierra Leone.
An estimated 70% of those infected have died in those countries.
The WHO officially declared Senegal Ebola-free on Friday.
Meanwhile, European Union foreign ministers are meeting in Luxembourg to discuss how to strengthen their response to the threat posed by Ebola.
European countries have committed more than 500m euros (£400m; $600m) but the UK is pressing to double that amount.
Ahead of the talks, German Foreign Minister Frank-Walter Steinmeier suggested the EU could send a civilian mission to West Africa that would serve as a platform for sending medical staff.
Another diplomat said there were plans for three countries to spearhead aid to the region - the UK for Sierra Leone, France for Guinea and the US for Liberia.
Earlier, the Spanish government said a nurse who became the first person to contract Ebola outside West Africa had tested negative for the virus.