The patient got sick at a town bordering Guinea, said Sidi Tunis, a spokesman at an Ebola response center in Sierra Leone.
Health officials are tracing anyone who may have had contact with the deceased.
The new case was confirmed Friday, hours after the World Health Organization gave the region an all-clear.
It made the proclamation after Liberia
, Sierra Leone
halted the chain of transmission simultaneously for the first time
The clearance was notable because it meant the three nations hardest hit by the Ebola outbreak had reported zero cases for at least 42 days.
Despite the clearance, health experts urged caution and vigilance, saying there may be some flare-ups.
And like Liberia, the virus has also resurfaced in Sierra Leone.
Liberia was first declared free of Ebola transmission in May, but the virus returned twice after that, with the latest case in November.
Before this week, Sierra Leone was declared free of Ebola transmission in November
while Guinea got its clearance last month
Forty-two days is twice the incubation period of the virus, a WHO requirement for a nation to be declared free of transmission.
The Ebola epidemic
killed about 11,300 people and infected 28,600 others, mostly in the three nations. It has devastated families, communities and economic systems since it emerged in late 2013 .
Evidence shows the virus can remain in the semen of male survivors for as long as a year. In rare instances, it can be transmitted to sexual partners.