The patient is a 10-year-old boy who lived in the Paynesville, a suburb east of the capital Monrovia, said the official, who declined to be named.
The case represents a setback for Liberia, which has seen more than 10,600 cases and 4,808 Ebola deaths since it was first announced in March, 2014, according to UN World Health Organisation figures.
The virus has killed about 11,300 people in Guinea, Sierra Leone and Liberia, but Sierra Leone was declared free of the virus on Nov. 7 and Guinea has begun its countdown to the end of the virus.
The 42-day countdown to be declared Ebola-free starts when the last patient tests negative a second time, normally after a 48-hour gap following their first negative test.
Fear of the virus transformed the three countries and hampered efforts in Sierra Leone and Liberia to recover from civil wars.
