The Oscar nominee was at the airport in Detroit on Friday when she spotted a soldier in uniform at her gate and then arranged to give her first class seat to the man, who had been ticketed in coach.
Jemele Hill, co-host of ESPN's Numbers Never Lie, was also on the flight and Tweeted about what happened next.
"Just saw actress Amy Adams do something incredibly classy," she wrote. "She gave her 1st class seat to an American soldier. I'm an even bigger fan now."
"I don't think he knows she did it," she added.
"As I was getting seated, I saw the flight attendant guide the soldier to Ms. Adams' seat. She was no longer in it, but it was pretty clear that she'd given up her seat for him," Hill told ABC News. "I was incredibly impressed."
Hill revealed that Adams, 39, whose father was in the U.S. military, briefly chatted to the soldier by the cockpit and then went back to her seat in coach.
"[She] did it so quietly and quickly that it speaks to her character," Hill said. "And somebody in coach just got a helluva seatmate."
Just saw actress Amy Adams do something incredibly classy. She gave her 1st class seat to an American soldier. I'm an even bigger fan now.— Jemele Hill (@jemelehill) June 27, 2014