Kelsey Zwick was flying with her 11-month-old daughter from Orlando to a children's hospital in Philadelphia last Thursday.
She was alone with the stroller, diaper bag and an oxygen concentrator because her daughter has a chronic lung disease.
A first-class passenger noticed she was in coach and offered her his first-class seat so she could be more comfortable.
"The flight attendant came over and she said 'Excuse me Ms. Zwick,' and I said 'Yes' and she said 'The man in 2D is waiting to switch seats with you,' so I'm just standing there looking at him, crying, just saying 'Thank you,'" Zwick said.
"She came walking up crying and said thank you and I said you're welcome and headed toward the back of the plane," Jason Kunselman said.
Zwick said she initially didn't get Kunselman's name until she posted about the kind deed on social media.
"Sooo... thank you. Not just for the seat itself but for noticing. For seeing us and realizing that maybe things are not always easy. For deciding you wanted to show a random act of kindness to US. It reminded me how much good there is in this world. I can't wait to tell Lucy someday. In the meantime... we will pay it forward. AA 588 passenger in seat 2D, we truly feel inspired by your generosity," Zwick wrote in the Facebook post.
American Airlines heard about the story and has since put the two in touch.