The World Health Organisation said the expanding monkeypox outbreak in more than 70 countries is an "extraordinary" situation that qualifies as a global emergency.
The WHO's declaration on Saturday could spur further investment in treating the once-rare disease and worsen the scramble for scarce vaccines.
WHO director-general Tedros Adhanom Ghebreyesus made the decision to issue the declaration despite a lack of consensus among members of the WHO's emergency committee.
It was the first time the chief of the United Nations health agency has taken such an action.
"In short, 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," Tedros said.
"I know this has not been an easy or straightforward process and that there are divergent views among the members" of the committee, he added.
Although monkeypox has been established in parts of central and west Africa for decades, it was not known to spark large outbreaks beyond the continent or to spread widely among people until May, when authorities detected dozens of epidemics in Europe, North America and elsewhere.
Declaring a global emergency means the monkeypox outbreak is an "extraordinary event" that could spill over into more countries and requires a coordinated global response.
The WHO previously declared emergencies for public health crises such as the COVID-19 pandemic, the 2014 West African Ebola outbreak, the Zika virus in Latin America in 2016 and the ongoing effort to eradicate polio.
The emergency declaration mostly serves as a plea to draw more global resources and attention to an outbreak.
Past announcements had mixed impact, given that the UN health agency is largely powerless in getting countries to act.
Last month, the WHO's expert committee said the worldwide monkeypox outbreak did not yet amount to an international emergency but the panel convened this week to re-evaluate the situation.
According to the US Centers for Disease Control and Prevention, more than 16,000 cases of monkeypox have been reported in 74 countries since about May.
To date, monkeypox deaths have only been reported in Africa, where a more dangerous version of the virus is spreading, mainly in Nigeria and the Democratic Republic of Congo.
In Africa, monkeypox mainly spreads to people from infected wild animals like rodents, in limited outbreaks that typically have not crossed borders.
In Europe, North America and elsewhere, however, monkeypox is spreading among people with no links to animals or recent travel to Africa.
As of July 19, Australia had reported 41 confirmed or probable cases of monkeypox, with the majority in NSW (22) and Victoria (15).
The WHO's top monkeypox expert, Dr Rosamund Lewis, said this week that 99 per cent of all the monkeypox cases beyond Africa were in men and that of those, 98 per cent involved men who have sex with men.
Experts suspect the monkeypox outbreaks in Europe and North America were spread via sex at two raves in Belgium and Spain.
Michael Head, a senior research fellow in global health at Southampton University, said it was surprising the WHO had not already declared monkeypox a global emergency, explaining that the conditions were arguably met weeks ago.
Some experts have questioned whether such a declaration would help, arguing the disease isn't severe enough to warrant the attention and that rich countries battling monkeypox already have the funds to do so; most people recover without needing medical attention, although the lesions may be painful.
"I think it would be better to be proactive and overreact to the problem instead of waiting to react when it's too late," Head said.