"There is one confirmed case," WHO spokesman Tarik Jasarevic told AFP, without providing further details.
The west African nation, where thousands died at the height of the epidemic last year, was first declared Ebola free in May, only to see the fever resurface six weeks later, and again in September.
Since the beginning of the outbreak, Liberia has registered more than 10,600 cases and 4,808 deaths, according to the latest situation report from the UN health agency published earlier this week.
Friday's news comes just days after the medical charity Doctors Without Borders (MSF) announced the last known Ebola case in Guinea, a three-week old girl, had recovered.
Guinea still must wait 42 days with no cases before being declared Ebola free.
Sierra Leone was declared to have beaten the virus earlier this month.