The WHO said that the confirmation of the disease by Tanzania's national public laboratory followed the death of five of eight people in Tanzania's northwest Kagera region who developed symptoms, which include fever, vomiting, bleeding and renal failure.
Among the dead was a health worker, the WHO said. The three who survived were getting treatment, with 161 contacts being monitored.
"The efforts by Tanzania's health authorities to establish the cause of the disease is a clear indication of the determination to effectively respond to the outbreak," said Matshidiso Moeti, WHO regional director for Africa.
"We are working with the government to rapidly scale up control measures to halt the spread of the virus."
With a fatality rate of as high as 88%, Marburg is from the same virus family responsible for Ebola and is transmitted to people from fruit bats. It then spreads through contact with bodily fluids of infected people.
Symptoms include high fever, severe headache and malaise which typically develop within seven days of infection, according to the WHO.
Equatorial Guinea is also battling its first-ever outbreak of Marburg that was confirmed in February.
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