Monkeypox spreads via close contact and tends to cause flu-like symptoms and pus-filled skin lesions.
So far this year, there have been more than 16,000 cases of monkeypox in more than 75 countries, and five deaths in Africa.
The viral disease has been spreading chiefly in men who have sex with men in the recent outbreak, outside Africa where it is endemic.
The WHO and national governments have been facing intense pressure from scientists and public health experts to take more action on monkeypox.
Dr. Tom Inglesby, the director of the Johns Hopkins Center for Health Security at the Bloomberg School of Public Health, says the WHO's declaration will accelerate the response to the unfolding epidemic.