The World Health Organization said there was a high risk the disease would spread.
Polio is a potentially deadly infection mainly affecting young people and can result in permanent paralysis.
It had been pushed back to just three countries until an outbreak was last week confirmed in Syria.
These are the first cases in DR Congo since 2012.
"WHO assesses the risk of further national spread of these strains to be high, and the risk of international spread to be medium," the WHO stated in its disease outbreak report.
The strain of polio involved comes from areas with poor vaccine coverage.
At least four cases have been identified.
Two were found in the eastern Maniema province and two further south in Haut-Lomami province, the WHO says.
A global vaccination campaign had left Nigeria, Pakistan and Afghanistan as the only countries where polio remained endemic.
The WHO had hoped to eradicate it by 2000 but this target has been repeatedly postponed.
DR Congo polio outbreak hits global eradication goal
14 June 2017
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Two outbreaks of polio have been identified in the Democratic Republic of Congo, in a blow to the goal of wiping out the disease from the world.
