The nation's health ministry on Saturday said a new case was confirmed on November 19, when blood samples from a patient admitted to a hospital the same week tested positive, CNN reported.
Two other high-risk contacts also tested positive, and all three are undergoing care at an Ebola treatment centre in the capital of Monrovia, the ministry said in a statement.
More than 40 contacts are being monitored, said Margaret Ann Harris, a spokeswoman for the World Health Organization (WHO).
"It is possible that we will find more cases," said Liberia's' Health Minister Bernice Dahn.
"The key is to stop it, find the source, and prevent the next one."
Liberia was first declared Ebola-free in May, but new cases later emerged.
Following another period of 42 days, two times the maximum incubation period, the WHO announced in September that Liberia was Ebola-free for the second time.
With 4,808 deaths from Ebola, Liberia had the highest number of fatalities from the outbreak, followed by Sierra Leone and Guinea.
More than 11,300 people died from the disease and 28,600 others were infected, mostly in the three countries, according to the latest WHO data.