The photo was released by Survival International, showing uncontacted Indians of the Ethno-Environmental Protected Area along the Envira river, in the Brazilian state of Acre, close to the border with Peru, photographed during a flight in May 2008.
The picture shows tribesmen, painted red head to toe, aiming longbows at the aircraft circling above.
The government foundation known as Funai said it photographed "strong and healthy" warriors, six huts and an area of crops. It is not known which tribe the group belonged to.
Funai does not make contact with indigenous people and prevents invasions of their lands, to ensure total autonomy for the tribes, the foundation said.
Fiona Watson, campaign co-coordinator for Survival International, said the tribesmen are likely "fragments" of what was once a much larger group.
Watson said contact with outsiders could even be fatal for these tribespeople, who would be vulnerable to diseases.
"These people are very isolated, they have no immunity to common things like the cold and flu and they could die very quickly," Watson told Newsnet.