Sierra Leone was celebrating last week when it discharged its last known Ebola patient from hospital.
News of the new case means the country is no longer Ebola-free. High-risk contacts of the woman have been identified, isolated and will now be watched for symptoms.
The National Ebola Response Centre is assessing whether to isolate the whole village of Sella in the Kambia district where the woman, who was in her mid-60s, died.
The end of the outbreak will only be declared six weeks after the last Ebola patient either dies or tests negative for the virus.
At the height of the outbreak, Sierra Leone was reporting more than 500 new cases a week.
Meanwhile, in Guinea there were three confirmed cases in the week up to 23 August.
The last known Ebola case in Liberia was discharged on 23 July.
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