Skarlatos, an infantryman who belongs to the Oregon National Guard, and his friends, Airman 1st Class Spencer Stone and Anthony Sadler, were on vacation when they encountered the gunman on the train.
The three friends rushed the gunman, who was armed with an AK-47, a handgun and a box cutter, subduing him.
Stone, who was injured in the attack, will be nominated for the Airman's Medal, Air Force Secretary Deborah Lee James announced Monday.
Both the Soldier's Medal and Airman's Medal are the highest awards given to service members who perform heroic acts in non-combat situations.
France honored all three men — Skarlatos, Stone and Sadler — with its Legion of Honor on Monday.
The gunman, identified as 26-year-old Ayoub El-Khazzani, is detained and being questioned by French counterterrorism police, the Associated Press reported. French and Spanish authorities said El-Khazzani is an Islamic extremist who may have spent time in Syria. His lawyer on Sunday said that he was homeless and trying to rob passengers on the train to feed himself, according to the AP.
Skarlatos, who has spoken out about the ordeal, said he didn't think twice about his actions, according to a news article by the National Guard.
"It wasn't a conscious decision," he said. "We didn't even have time to think about it. We just acted."
On the train, Skarlatos noticed the gunman and woke up Stone, who was asleep in the seat next to him, said his stepmother, Karen Skarlatos.
"Alek hit his buddy Spencer on the shoulder and said, 'Go, go, go!'" she said. "They're long-time buddies. They were thinking just alike."
Stone got to the gunman first and took away the attacker's AK-47, Karen Skarlatos told Military Times.
Alek Skarlatos then wrested a handgun away from the gunman, who then pulled out a box cutter or knife of some sort, she said.
"Spencer was starting to get nailed pretty good with that, so Spencer got a choke hold on the guy, and Alek took the butt of the AK did some nice work on the guy's head, and they rendered him unconscious," she said.
After the gunman was subdued, Stone attended to a passenger who had been shot and was bleeding, Karen Skarlatos said.
"He just stuck his fingers right in the hole in the guy's neck and was able to stop the bleeding on the artery that was spurting blood," she said.
Thankfully, Karen Skarlatos heard from her stepson about the incident instead of seeing it on the news first, she said.
"I know Alek is brave and strong and gutsy, and if someone would have said, 'Who's the guy that would jump up in this whole town and save the day?' I would say, 'That's my boy.' "
Senior Army leaders praised Skarlatos for his actions.
"Spc. Skarlatos' actions that day epitomize what we mean by a soldier of character — one who lives by a personal code where dedication to duty and taking care of others is sacred," said Army Secretary John McHugh in a statement. "His actions, and those of his fellow serviceman and passengers, exemplify the highest standards of selfless service. We are proud to count him in our ranks."
Army Chief of Staff Gen. Mark Milley agreed, commending Skarlatos for his "heroic actions that saved hundreds of lives."
"I am proud to call you a hero and a soldier," Milley said in a statement.
Michelle Tan is the editor of Army Times and Air Force Times. She has covered the military for Military Times since 2005, and has embedded with U.S. troops in Iraq, Afghanistan, Kuwait, Haiti, Gabon and the Horn of Africa.