Boero, 38, suffered the accident on board his RR 450 Beta motorbike close to the sand dunes of the Atlantic seaside city of Necochea in Buenos Aires province.
He suffered serious injuries to his thorax and cardiac arrest.
Eyewitness Federico Sepulveda, 14, the son of a farmer, told AFP: "I saw him reach a hundred feet away from me. He was driving very, very fast. And suddenly I saw the bike destabilise.
"He veered to the left and struck an embankment on the edge of a field. He went over his bike [and] rolled several times before coming to rest.
"I do not understand what happened: the road was very flat, slightly sandy with some stones and straight."
He was two kilometres from the finish of the 57km special stage held between Necochea and Energia, won by Chile's Francisco Lopez.