All the victims were people who had had primary contact with Patrick Sawyer, who collapsed on arrival at Lagos airport on July 25 and later died, health minister Onyebuchi Chukwu said.
A nurse who treated Mr Sawyer not knowing what condition he had and without protective gear also died.
Mr Chukwu said: "As at today, 77 primary and secondary contacts of the index case have been placed under surveillance or isolation."
The latest case was another nurse who had had primary contact with Mr Sawyer.
When she fell ill, she was put into isolation and tested for the disease over the weekend.
She had been at home with her husband, who was also now under surveillance, Mr Chukwu said.
The West African Ebola outbreak is the worst in history and the World Health Organisation (WHO) said on Friday it represents an international health emergency that will likely continue spreading for months.
It said 961 people have died during the outbreak and 1779 have been infected.
The disease has strained health systems in affected states and governments in Sierra Leone, Liberia and Nigeria have responded with measures including declaring national emergencies.
Nigeria faces the additional problem that public doctors are on strike over pay and working conditions and have resisted calls by the government to end their strike to tackle the unfolding Ebola crisis.
The Nigerian Red Cross said it had provided 18 volunteers to work with the authorities to educate people on how Ebola is spread.
Ebola is one of the world's most deadly diseases, with no known vaccine or cure.
The Zaire strain of the disease - the one currently spreading through West Africa - can kill up to 90 per cent of sufferers, although in the latest outbreak the death toll has been around 55 per cent.