The Indians were sighted in an Ethno-Environmental Protected Area along the Envira River in flights over remote Acre state, said the Brazilian government foundation, known as Funai.
Funai said it photographed "strong and healthy" warriors, six huts and a large planted area. But it was not known to which tribe they belonged, the group said.
"Four distinct isolated peoples exist in this region, whom we have accompanied for 20 years," Funai expert Jose Carlos Meirelles Junior said in a statement.
The tribe sighted recently is one of the last not to be contacted by officials. Funai does not make contact with such tribes Indians and prevents invasions of their land to ensure their autonomy, the foundation said.
Survival International said the Indians are in danger from illegal logging in Peru, which is driving tribes over the border and could lead to conflict with the estimated 500 uncontacted Indians now living on the Brazilian side.
There are more than 100 uncontacted tribes worldwide, most of them in Brazil and Peru, the group said in a statement.
"These pictures are further evidence that uncontacted tribes really do exist," Survival director Stephen Corry said.
"The world needs to wake up to this, and ensure that their territory is protected in accordance with international law. Otherwise, they will soon be made extinct."