The scientists claim to have traced the river's source to a snow-capped mountain in southern Peru, adding a new twist in the swirling debate over the longest river label.
The Amazon is considered the world's largest river by volume, but scientists have believed it is slightly shorter than Africa's Nile.
The Brazilian scientists' 14-day expedition extended the Amazon's length by about 176 miles (284 kilometers), making it 65 miles (105 kilometers) longer than the Nile.
According to the team's results, which have not been published, the Amazon is 4,225 miles (6,800 kilometers) long. The Nile stretches 4,160 miles (6,695 kilometers).
"Today, we can consider the Amazon the longest river in the world," study author Guido Gelli, director of science at the Brazilian Institute of Geography and Statistics, told London's Telegraphnewspaper.
The Amazon and the Nile have been at the center of a centuries-old rivalry over the "world's longest" title.
In the 20th century, consensus gave the title to the Nile.
Determining the length of a river is tricky because scientists have to pinpoint both where the river begins and ends, Andrew Johnston, a geographer at the Smithsonian's National Air and Space Museum in Washington, D.C., told National Geographic News.
Though Johnston was not a part of the research team, he is familiar with the Amazon mapping effort. He participated in related research in 2000, which was funded in part by the National Geographic Society. (National Geographic News is part of the National Geographic Society.)
"The mouth of the Amazon is so wide, it's hard to pick the exact spot at which to choose the end point with that kind of precision," he said.
"Personally, I would want to know a little bit more about how they came to that number before I was comfortable saying, 'Yes this is longer,'" he added.
Glenn Switkes is the São Paulo-based Latin America coordinator for the conservation group International Rivers Network.
The debate over river length is trivial, he said, but he hopes the announcement will focus international attention on the Brazilian government's efforts to build more than 60 large dams on the Amazon's major tributaries.
"The Amazon continues to be by far the river system which sustains the most biodiversity and which continues to maintain a natural state throughout most of its course," he said.