The 30-year-old woman passed away in a hospital in a densely-populated area of the capital, Monrovia.
The World Health Organization (WHO) declared that Liberia was free of Ebola in January, but there have been small flare-ups of the disease in the region.
In neighbouring Guinea, several cases have been identified in recent weeks.
More than 11,300 people died over the past two years in the world's worst Ebola epidemic, almost all of them in Liberia, Sierra Leone and Guinea.
The infected woman was brought to a clinic in Paynesville, just east of Monrovia, after falling ill, officials said. She was later transferred to a hospital in the capital, but died on Thursday.
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A team has been sent to the woman's community and the clinic where she was being treated to investigate the case and identify people who may have been in contact with her, the WHO said in a statement.
Earlier this week, the agency had said the Ebola outbreak no longer constituted an international public health risk, but it warned that flare-ups were expected, largely due to the virus presence in survivors.
In January, the WHO declared the "end of active transmission" in Liberia after 42 days without a new case.
A woman in Liberia has died of Ebola, months after the epidemic was declared over in the country, health officials say.