The classification is the highest alert that the WHO can issue and follows a worldwide upsurge in cases.
It came at the end of the second meeting of the WHO’s emergency committee on the virus.
More than 16,000 cases have now been reported from 75 countries, said WHO director general Dr Tedros Adhanom Ghebreyesus.
There had been five deaths so far as a result of the outbreak, he added.
There are only two other such health emergencies at present – the coronavirus pandemic and the continuing effort to eradicate polio.
Dr Tedros said the emergency committee had been unable to reach a consensus on whether the monkeypox outbreak should be classified as a global health emergency.
However, he said the outbreak had spread around the world rapidly and he had decided that it was indeed of international concern.
Too little was understood about the new modes of transmission which had allowed it to spread, said Dr Tedros.
“The WHO’s assessment is that the risk of monkeypox is moderate globally and in all regions, except in the European region, where we assess the risk as high,” he added.
There was also a clear risk of further international spread, although the risk of interference with international traffic remained low for the moment, he said.
Dr Tedros said the declaration would help speed up the development of vaccines and the implementation of measures to limit the spread of the virus.
The WHO is also issuing recommendations which it hopes will spur countries to take action to stop transmission of the virus and protect those most at risk.
“This is an outbreak that can be stopped with the right strategies in the right groups,” Dr Tedros said.
What is Monkeypox?
James Gallagher explains what it is and if we should be worried.
Monkeypox was first discovered in central Africa in the 1950s.
In the UK, there have so far been more than 2,000 confirmed cases.
Health officials are already recommending people at highest risk of exposure to monkeypox – including some gay and bisexual men, as well as some healthcare workers – should be offered a vaccine.
Initial symptoms typically include a high fever, swollen lymph nodes and a blistery, chickenpox-like rash or lesions – often on the mouth or genitals in the recent cases. Infections are usually mild.
Declaring a global emergency is a significant act.
It is a rallying cry for countries to take the virus seriously, it raises awareness around the world and it can help poorer countries get the tools they need to control monkeypox.
In principle, we have the tools to stop the virus. Monkeypox does not spread as easily as Covid and we already have a vaccine (developed for smallpox) that offers good protection.
And while anyone can catch monkeypox, the outbreak is overwhelmingly concentrated in gay and bisexual men, as well as other men who have sex with men.
This can make the outbreak easier to tackle, as efforts, including vaccines and public health information, can be targeted at those most at risk.
But remember, there are still countries where same-sex relationships are illegal – and stigma and persecution can act as a barrier to help.
Whether we can stop monkeypox can be stopped is as much a societal and cultural challenge as it is about the virus.
