Volodymyr Zelensky (L) and his wife Olena during a presidential election debate in Kiev. Fiel / AFP
Zelenska, 42, added that her husband, President Volodymyr Zelensky, and their two children tested negative.
"Today I've received a positive COVID-19 test result," she said on Facebook.
On Thursday, Ukrainian authorities said there had been an "alarming" rise in coronavirus cases as the country eases lockdown restrictions.
On Friday, the ex-Soviet country reported 29,753 cases and 870 fatalities. The Ukrainian president urged officials to urgently establish the reason behind a spike in new cases.
"Only the observation of security measures will protect us from the threat of getting sick and save our economy from the quarantine," he said.
President Volodymyr Zelensky and Olena Zelenska attend the enthronement ceremony of Japan's Emperor Naruhito in Tokyo. Reuters
The first lady said that she felt fine and was self-isolating. She said the positive test was a surprise as she and her husband have sought to follow social distancing rules and protected themselves.
But the 42-year-old leader came under fire last week when he appeared without a mask in a cafe during a visit to the city of Khmelnytsky in central Ukraine.
Some experts have said the uptick is due to an increase in testing.
Lockdown measures eased gradually in late May and early June with a resumption of public transport, including metro systems and long-distance and local train services.
Ukraine launched domestic flights on June 5 and will resume international flights on Monday.
Elsewhere in Europe, frustrations with COVID-19 curbs were spilling over, with scuffles breaking out at a large anti-restrictions protest in the German city of Kassel, and thousands joining a similar demonstration in Liestal, Switzerland.
England's months-old rules have been relaxed thanks in large part to a successful vaccination drive, enabling outdoor gatherings of up to six people, or two households, in what newspapers have dubbed "Happy Monday."
The global death toll from the virus surged past 106,000 on Saturday, with the United States quickly becoming the epicentre of the pandemic that first emerged in China late last year.
Some intensive care specialists are trying to hire more permanent staff. Others want to create a reservist "army" of medical professionals ready to be deployed wherever needed to work in wards with seriously ill patients.
According to a government statement 1,587 fatalities in the last 24 hours, lowest in the last two months. On June 15, India recorded 60,461 cases, lowest since March 29.
The attack is the third mass kidnapping in three weeks in northwest Nigeria, which have authorities have attributed to armed bandits seeking ransom payments.
Police in Ghaziabad in northern Uttar Pradesh state this week registered a case after a video shared widely by Twitter users this week showed a few men, apparently Hindu, beating an elderly man believed to be a Muslim and cutting his beard.