A random act of kindness has left a mother – and the rest of the world – in tears.

Kelsey Rae Zwick traveled with her eleven-month-old daughter Lucy from Orlando to Philadelphia for treatment at the Children's Hospital in Philadelphia, when a complete stranger made a touching gesture.

In a Facebook post, Kelsey said she and baby Lucy, who suffers from chronic lung disease, have boarded American Airlines Flight 588.

A flight attendant spoke to her and informed her that a first-class passenger was about to change his seat.

Kelsey was incredibly touched by the sweet gesture and wrote, "To the man in 2D. Today you traveled from Orlando to Philly.

"I do not know you, but I imagine you saw us somewhere. I pushed a pram, had a diaper bag on my arm and also brought an oxygen machine for my daughter.

The flight attendant came and told me that you were waiting for the seats to change. You have given up your comfortable, first-class seat for us. I could not hold back the tears and cried when my daughter Lucy laughed.

Kelsey Rae Zwick

"We had a smile on our faces when we wanted to see their friends at the CHOP (Philadelphia Children's Hospital).

"We got on the plane, made ourselves comfortable in our window seat, and joked that we had to sit next to my screaming but happy baby.

"The flight attendant came by and told me you were waiting to change place. You have given up your comfortable, first-class seat for us. "

The touching gesture that Kelsey said she "can not wait to tell Lucy one day," she said in tears.

She said, "I could not hold back the tears, I cried the corridor while my daughter Lucy laughed!"

Kelsey, unable to properly thank the Good Samaritan as they passed each other in the corridor, shared the story on Facebook to find the man.

    Kelsey is trying to track down the AA588 passenger in Seat 2D on their American Airlines flight yesterday

Getty – contributor

Kelsey is trying to track down the AA588 passenger in Seat 2D on their American Airlines flight yesterday

The Post found one hit among the social media users and collected over 349,000 likes, and many people were in tears next to the grateful mother.

One person wrote, "To have been seen by this man and to stretch so calmly, but meaningfully, pulls each of my chains of hearts absolutely."

Another wrote, "Oh wow. It brought tears to my eyes, knowing that there are good people in the world. "

It does not seem that Kelsey has yet found the mysterious man, but in the meantime she says she promises to "pay it forward".

She said, "It reminded me how much good in this world is.

"In the meantime … we will continue to pay it. AA588 passenger in Seat 2D, we really feel inspired by your generosity. "

This story had previously appeared on News.com.au and was released here with permission.

A mother had been in tears when her baby did not stop screaming on the plane … but a friendly flight attendant saved the day the best way.

This touching moment stands in stark contrast to a recent study that found that millions of Britons had never experienced a friendliness act from a stranger.

The Script surprise fans with FREE tickets on the Random Acts of Kindness Day