The shape of the scratch marks suggested the beast was swimming
The 15m (50ft) trackway that reveals one animal's underwater odyssey was discovered in the Cameros Basin in Spain, once a vast lake.
The S-shaped prints suggest the beast clawed at sediment on the lake floor as it swam in about 3m (10ft) of water.
The marks are about 125 million years old, dating to the Early Cretaceous, the team writes in the journal Geology.
They were left by a large, bipedal, carnivorous dinosaur.
"We came across them about three or four years ago," explained Dr Loic Costeur, a palaeontologist at the University of Nantes, France, and a co-author of the paper.
"The Cameros Basin has thousands of walking footprints from diverse dinosaur fauna, but when we saw these it was obvious straightaway that this was a swimming dinosaur."
The underwater trackway, which is well-preserved in sandstone, is made up of 12 consecutive prints each consisting of two to three scratch marks.
"The footprints are really peculiar in their shape and morphology - they are not at all like walking footprints," Dr Costeur told the BBC News website.
"In walking footprints, you can recognise the shape of the foot; but here it is not at all the case: it is sets of grooves on the sediment surface.
"You get the idea that the animals' body was supported by water as it was scratching the sediment."
Ripple marks around the track suggested the dinosaur was swimming against a current, attempting to keep a straight path, the team said.
Further investigation of the well-preserved track revealed more about the beast's swimming style.
"The dinosaur swam with alternating movements of the two hind limbs: a pelvic paddle swimming motion," said Dr Costeur.
"It is a swimming style of amplified walking with movements similar to those used by modern bipeds, including aquatic birds."
For many years, the question of whether dinosaurs were able to swim remained unanswered.
Investigations into dinosaur anatomy and ecology suggested it was possible, but very little hard evidence existed documenting this behaviour.
But Dr Costeur described the find as "extremely exciting" and said it provided the first compelling evidence that dinosaurs were able to swim.
"The trackway at La Virgen del Campo opens the door to several new areas of research," said Costeur.
"New biomechanical modelling will increase our understanding of dinosaur physiology and physical capabilities, as well as our view of the ecological niches in which they lived."