The 24-year-old man who died had developed a high fever, diarrhoea and abdominal pains, and was vomiting blood.
After initially being treated for malaria, he was diagnosed as having contracted the Sudan strain of the Ebola virus.
"As of today, we have seven confirmed cases, of whom we have one confirmed death," Kyobe Henry Bbosa, Ebola incident commander at the Ugandan Ministry of Health, told a briefing.
"But also we have seven probable cases that died before the confirmation of the outbreak."
Bbosa said their investigations had not yet pinned down the index case or "patient zero" but said the outbreak appeared to have started around the beginning of September "when people started dying" in a small village in Mubende district in central Uganda.
Uganda last reported an outbreak of the Ebola Sudan strain in 2012.
The strain is less transmissible than Ebola Zaire, Patrick Otim, health emergency officer at the World Health Organisation Regional Office for Africa said at the briefing, adding that it had a lower fatality rate in previous outbreaks.
However, Otim said the Sudan strain posed a greater threat because the world did not yet have a vaccine for it like it has with the Zaire strain.
"So it's very, very critical that at this point we treat this outbreak as serious because we may not have the advantage that we have gained in terms of advancement in medical countermeasures," he said.
Mubende lies on a highway that starts from the capital Kampala, connecting Uganda to Democratic Republic of the Congo, and Otim said this meant there was a threat of cross border transmission.
In 2019, Uganda experienced an outbreak of Ebola Zaire.
The virus was imported from neighbouring DR Congo which was battling a large epidemic in its northeastern region.
In August, a new case of Ebola virus was confirmed in the city of Beni in eastern DR Congo.
