“The main reason for this declaration is not what is happening in China but what is happening in other countries,” said WHO chief Tedros Adhanom Ghebreyesus.
The concern is that it could spread to countries with weaker health systems.
The death toll is now at 170 in China.
The WHO said there had been 98 cases in 18 counties outside of China, but no deaths.
There have been eight cases of human-to-human infection – in Germany, Japan, Vietnam and the United States.
Dr Ghebreyesus, speaking at the press conference in Geneva, described coronavirus as an “unprecedented outbreak” that has been met with an “unprecedented response”.
He praised China’s “extraordinary measures” taken to prevent it from spreading.