Palaeontologist Kenneth Lacovara of Drexel University in Philadelphia, who led the effort for its excavation and analysis, said they calculated its weight on the basis of the bones in its upper arm and thigh.
Dreadnoughtus weighed 65 tonnes and measured 26 meters with a neck more than 11 meters long and a tail almost 9 meters long.
Tipping the scales at seven times heavier than a T-Rex, it makes the North American menace that also lived during the Cretaceous Period look puny.
Dreadnoughtus had "the largest reliably calculable weight" of any known land animal - dinosaur or otherwise, Lacovara said.
Another giant Argentine dinosaur, Argentinosaurus, might have been larger, but its scant remains do not allow a reliable weight estimate. Another group of scientists in May had cited Argentinosaurus, with an estimated weight of 90 tons as the largest dinosaur.
While strictly a vegetarian, Dreadnoughtus was no pansy. With its size and a tail that could have clobbered any predator foolish enough to attack it, it probably had nothing to fear from even the largest meat-eating dinosaurs.
Its name reflects that.
"We decided on Dreadnoughtus - meaning 'fearer of nothing' - because when you're as big as this thing was, you're probably not afraid of too much," said one of the researchers, Matt Lamanna of the Carnegie Museum of Natural History in Pittsburgh.
Lacovara said the name also was a nod to the powerful battleships called dreadnoughts, dating from the turn of the last century, that were designed to be impervious to attack.
Dreadnoughtus probably spent its days munching massive quantities of plants to fuel its enormous body. It was a member of a group of dinosaurs called titanosaurs that are thought to have been the largest dinosaurs that ever lived.
Argentinosaurus also was a titanosaur.
Most titanosaurs are known only from fragmentary remains, but the scientists found 45 percent of the skeleton of Dreadnoughtus, including most of the important bones.
The researchers found two specimens of Dreadnoughtus side by side - one larger than the other, but the scary thought is that they believe the larger one was not even fully grown.
The study appears in the Nature journal Scientific Reports.
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