North Korean leader Kim Jong Un convened an emergency politburo meeting after the person returned with symptoms of coronavirus infection, KCNA reported on Sunday.
The state news agency did not specifically mention whether the individual had been tested, but 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.
The person had defected to South Korea three years ago, and returned across the fortified border that divides the two Koreas with symptoms that suggested COVID-19.
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," KCNA reported.