The strain of polio, WPV1, was believed to have been imported from Pakistan, where the crippling virus is endemic, as it is genetically linked to the same type circulating there, BBC reports.
So far, nine cases have been confirmed in China, six children and three adults, all in Hotan prefecture in the western province of Xinjiang, WHO spokesman Oliver Rosenbauer said, Daily News and Analysis reports.
Xinjiang province is on the border of Pakistan.
The outbreak marks the latest setback to a global campaign to eradicate polio, now endemic in only four countries -- Afghanistan, India, Pakistan and Nigeria, DNA reports.
The WHO had already warned of a high risk of the virus spreading further during Muslim pilgrimages to Mecca, BBC reports.
Polio (also called poliomyelitis) is highly infectious and affects the nervous system, sometimes resulting in paralysis.
It is transmitted through contaminated food, drinking water and faeces.
The Chinese authorities are now investigating the cases, which had been detected over the past two months, and a mass vaccination campaign is underway.
"So far all the right things are being done," WHO spokesman Oliver Rosenbauer told Reuters news agency.
Polio was last brought into China from India in 1999. China's last indigenous case was in 1994.
WHO officials had issued warnings about the possibility of the virus spreading within the country to uninfected areas.
However, to eradicate polio from Pakistan each child would need to be given oral vaccines.