The head of the World Health Organization made the announcement Saturday despite the fact that an emergency committee failed to reach a consensus.
"We have an outbreak that has spread around the world rapidly, through new modes of transmission, about which we understand too little, and which meets the criteria in the international health regulations," the director general of the WHO said.
The global monkeypox outbreak is a public health emergency of international concern.
A month after its first meeting, an emergency committee did not reach a decision on the matter.
As of this week, more than 16,000 cases have been reported in 75 countries and territories, with five deaths.
The head of the international health organization acted as a tie-breaker by going ahead with the declaration because of the international health regulations.
The declaration of an international health emergency was supported by nine of the committee's members. Three people voted for and 11 against.
The story is evolving and will be updated.