Spencer Stone, hailed as a hero after an attack on a French train, has been stabbed in Sacramento, California.
He was attacked while defending a friend, an Air Force official told the BBC. A hospital official says his condition is "serious".
Police say two men attacked Mr Stone in the early hours of Thursday morning. One man stabbed him before fleeing.
Ken Bernard of the Sacramento police said that the incident is not related to the train attack.
"This is not a terrorism related incident nor is it related to what happened in France months ago," Deputy Bernard said.
Stone had been visiting night clubs in the area with four friends before the stabbing took place. Authorities believe that alcohol may have been a factor.
"The assault does not appear to be a random act," Deputy Bernard said. "Its believed to be related to a night club incident."
Mr Stone, 25, is one of three Americans that helped to thwart an attack on a French train in August.
They were hailed as heroes for tackling a man who was carrying a Kalashnikov rifle, an automatic pistol with ammunition clips and a box cutter knife.
The men, assisted by other passengers, threw him to the floor of the carriage, removed his guns and restrained him.
Mr Stone was slashed by the attacker and later required surgery to reattach part of his hand.
He and the two others, Alek Skarlatos and Anthony Sadler, were hailed by French President Francois Hollande and greeted as heroes upon their return to the US.
They received awards from Secretary of Defense Ashton Carter and met President Barack Obama at the White House.
Mr Stone is currently a serving member of the Air Force with the rank of Airman 1st class.
Mr Skarlatos tweeted "Everybody send prayers out to the Stone family today".