The man illegally crossed the border this month, state media said on Sunday.
If confirmed, it would be the first case officially acknowledged by North Korean authorities, who have so far said the country has no confirmed cases of the new coronavirus.
Kim declared a state of emergency and imposed a lockdown on the border city of Kaesong, calling it a “critical situation in which the vicious virus could be said to have entered the country,” state news agency KCNA reported.
According to KCNA, a person who defected to South Korea three years ago returned across the fortified border that divides the two Koreas with symptoms that suggested COVID-19.
“An emergency event happened in Kaesong City where a runaway who went to the south three years ago, a person who is suspected to have been infected with the vicious virus returned on July 19 after illegally crossing the demarcation line,” KCNA said.
KCNA did not specifically mention whether the individual had been tested.
But it said an “uncertain result was made from several medical check-ups of the secretion of that person’s upper respiratory organ and blood,” prompting officials to quarantine the person and investigate anyone he may have been in contact with.
North Korea has received thousands of coronavirus testing kits from Russia and other countries, and imposed strict border closures.
Thousands of people in North Korea were quarantined, but restrictions had recently eased.