Mark Moogalian, a 51-year-old professor at the Sorbonne, was the first passenger to wrestle with Ayoub El-Khazzani.
Moogalian, who lives in Paris but is originally from Midlothian, Virginia, in the United States, is the previously unnamed man who came to the aid of "Damien A", 28, a French banker who confronted El-Khazzani.
The academic acted instinctively to protect his wife, Isabella Risacher, who was also on board the Thalys train.
He grabbed the assault rifle off El-Khazzani, who then drew a sidearm and shot him in the neck before taking back the rifle, his sister has revealed.
Three other US citizens including two military personnel, and Chris Norman, a British businessman, then stepped in to overpower the assailant.
Moogalian's sister Julia said: "He made sure his wife was hidden behind a seat. He did manage to get the weapon away from the gunman. But the gunman then pulled another gun and shot my brother ...
"The French President has reached out to ask him to come to dinner when he is fit enough."
The keen musician and cyclist was shot in the back of the neck and the bullet passed through his body causing nerve damage, his sister said.
"We are extremely proud of him."