"This is one of the biggest in the world and one of the most complete of these giants that exist," said Jorge Calvo, director of the palaeontology centre at the National University of Comahue.
Since the first bones were found on the banks of Lake Barreales in the Argentinian province of Neuquen in 2000, palaeontologists have dug up the dinosaur's neck, back region, hips and the first vertebra of its tail.
Peter Mackovicky, associate curator for dinosaurs at Chicago's Field Museum, said: "I'm pretty certain it's a new species."
"I've seen some of the remains of Futalognkosaurus and it is truly gigantic," he said.
Patagonia also was home to the other two largest dinosaur skeletons found to date - Argentinasaurus, at around 35m, and Puertasaurus reuili, between 35m and 40m long.
Comparing the three herbivores, however, is difficult because scientists have found only few vertebrae of Puertasaurus and while the skeleton of Futalognkosaurus is fairly complete, scientists have not uncovered any bones from its limbs.
The fossil was 70 per cent preserved, compared to about 10 per cent for other giant dinosaur finds in the world.