Health ministry spokesman Carlos Alberto said a girl younger than two years died on Sunday in Uige, the epicentre of the epidemic north of the capital, Luanda.
"This baby was the child of a 19-year-old woman who died on Sunday morning and the baby died the same evening," he said.
The most serious recorded outbreak of the disease was in the Democratic Republic of Congo between 1998 and 2000, when 123 people died.
A severe form of haemorrhagic fever akin to Ebola, the Marburg virus was first identified in 1967. The disease can spread on contact with body fluids such as blood, urine, excrement, vomit and saliva.
It kills around one in four who contract it, and a specific treatment is unknown.
Alberto said affected parents or children usually both die.
"Generally, that's been the pattern in Uige; either the children die first and the parents follow suit or vice-versa due to the .... contact they have had with each other."
Three-quarters of the deaths have been children under five, according to the World Health Organisation (WHO), but the virus has also started to claim adult victims, including at least seven medical workers.
Alberto said many victims died because they consulted 'kimbandeiros', or traditional healers, and only came to hospital when it was too late.
"For example, the last nurse to die in Uige was first taken to a kimbandeiro and only when her condition worsened was she brought to hospital."
Luanda's provincial health director Vita Mvemba said the "condition in the capital city was stable", with no more deaths.
Of the seven people who contracted Marburg in Luanda, two have died an Italian doctor and a 15-year-old boy who both came from Uige.
The epidemic has so far claimed the lives of two foreign doctors; one Italian and the other Vietnamese.
The epidemic broke out in October 2004 but has worsened in recent weeks.