The emergence of a new reserve in the Espirito Santo field comes a month after a reserve of up to eight billion barrels was found nearby.
No figures have been put on the size of the latest discovery although Petrobras said it offered "high potential".
It believes Brazil could become one of the world's top 10 oil producers.
Brazil currently has proven oil reserves of 14 billion barrels, more than half of which have been discovered in the past five years.
Ministers believe a succession of recent finds could enable Brazil to eventually match the oil output of powerhouses such as Venezuela and Saudi Arabia.
"The discovery confirms the Espirito Santo Basin's high potential for gas and light oil, which is expected to result in increased recoverable volumes from the area," Petrobras - which holds a 65% stake in the firm exploring the basin - said.
Light oil is much prized because it is generally of a high quality and cheaper to refine.
Petrobras shares rose more than 4% on the Sao Paulo Stock Exchange in response.