"A woman has been confirmed as an Ebola patient… This is a new case after we have gone more than 27 days without a single case. It is a setback,” government spokesman Lewis Brown told the news agency AFP.
It was not immediately clear where the woman became infected.
Liberia was hit the hardest by the Ebola outbreak in West Africa; the country's death toll currently totals more than 4,000.
Liberia's last known Ebola patient, Beatrice Yardolo, was discharged from a treatment center in Monrovia on March 5; it added to hopes that Liberia could be on its way to being officially announced Ebola-free.
Earlier this month, the World Health Organization said that no new Ebola case had been registered in Liberia since February 19.
According to the WHO, at least 42 days should pass from the last known case of Ebola before a country can be declared virus-free.
More than 10,000 people have already been killed by the Ebola virus since the outbreak was registered in Guinea in December 2013; the virus then spread to neighboring Liberia and Sierra Leone, where it has yet to be eradicated despite a huge international response.
Currently, there is no proven vaccine to prevent the Ebola infection; researchers are speeding up efforts to resolve the problem.