The World Health Organization said the new case was the aunt of 22-year-old Marie Jalloh, who died of Ebola on January 12.
The 38-year-old woman "was a primary caregiver during (her niece's) illness," WHO spokesman Tarik Jasarevic told AFP in an email.
He added that the patient had developed symptoms on yesterday while she was being monitored at a quarantine facility.
"The patient is being treated now," Jasarevic said.
Sierra Leone's health ministry spokesman Sidi Yahya Tunis also confirmed the new case, saying the aunt had helped wash Jalloh's body to prepare it for an Islamic burial.
Ebola is at its most infectious as people are dying or in the bodies of those who have died from the virus.
After Jalloh's case was confirmed last week, officials in Sierra Leone said 100 people she had contact with had been quarantined, sobering news that came just days after the region thought it had overcome the devastating epidemic.
Officials in Freetown have urged the public not to panic and to work together to avoid any further spread.
A week ago, WHO announced the end of viral transmission for the outbreak which began in December 2013 and killed more than 11,300 people, mainly in Guinea, Liberia and Sierra Leone.
Jasarevic told AFP that Sierra Leone has taken all the necessary steps following the new flare-up.
"It is a concrete demonstration of the government's strengthened capacity to manage disease outbreaks," he said.
Tunis said the latest patient had been taken from her home in the central city of Magburaka to a military hospital in the coastal capital, Freetown, which is fully equipped to handle Ebola cases.
He added that screenings, including checkpoints at major roads, were being installed to check for those displaying signs of the virus, which is spread via the bodily fluids of recently deceased victims or carriers showing symptoms.
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