US airman Spencer Stone, who helped subdue an attacker on a French train in August, is in a stable condition after being stabbed in California, a US air force spokesman said.
Mr Stone is being treated in a hospital in the Sacramento area, said Lt Col Christopher Karns, who added that details are sketchy.
Mr Stone is assigned to Travis air force base in California.
Lt Col Karns said the matter is being handled by local law enforcement officials.
He had no details on the nature or exact location of the stabbing.
Mr Stone, 23, and two of his childhood friends from Sacramento, Alek Skarlatos and Anthony Sadler, stopped a gunman on a Paris-bound passenger train in August, receiving worldwide accolades and gaining sudden celebrity.
President Barack Obama met with the three last month, praising them for their quick thinking and courage and calling them “the very best of America”.
They were also awarded France’s highest honour by President Francois Hollande.
The three have appeared on late-night talk shows and received a hometown parade in their honour since subduing the attacker while on holiday in Europe.
Mr Stone, had undergone medical treatment at a military base in Germany for a lacerated thumb and other injuries he suffered in the attack.