Antonelli, just 25 years of age, crashed during the opening lap of the rain-soaked race, which took place at Moscow Raceway.
It appears he lost control of his Kawasaki motorcycle on the straight between turns 14 and 15. He eventually fell off the bike, only to be hit in the head by the bike of another rider, believed to be fellow Italian Lorenzo Zanetti.
He was treated by an emergency medical team but succumbed to his injuries shortly after at Moscow Raceway’s medical center.
Both the Supersport World Championship race as well as the day’s headline Superbike World Championship race were canceled out of respect.
The tragic news comes just a week after the death of another rider, Bill Warner, during a time trial event in northern Maine.