Fabrice Miguet, a 49-year-old from Normandy, France, was injured in the Superstock race on Saturday.
Organisers of the the grand prix, held every year at Dundrod Circuit in Northern Ireland, said he underwent surgery but that “the medical expectation is that he has sustained injures that are un-survivable”.
“Our thoughts are with Fabrice’s family, team and many friends at this difficult time,” they said in a statement.
Mr Miguet came off his bike at the Joey’s Windmill section of the track during lap four, the Belfast Telegraph reported.
An English rider, Davey Todd, was injured in a separate incident and suffered suspected fractures. He was sent to hospital but later released.
The newspaper said a spectator was taken to hospital too, after hitting a bike rider while trying to take a photo.
The race was abandoned just after 5pm on Saturday due to poor weather.
The incident comes after William Dunlop was killed in a crash during a practice session at the Skerries 100 road races in County Dublin last month.