Megan Shufflebarger from Indiana was shopping with her two-year-old daughter Kinley for birthday presents, when a teenager asked which doll was Kinley's favourite.
After Kinley pointed out which one "she lubs", the teenager walked away and at first Shufflebarger thought that was to be the end of their friendly exchange.
Shufflebarger said while in the Target store, her daughter Kinley had been walking around making a list of things she wanted for her birthday, "oohing and aahing" at the toys.
The teenager overheard her and approached her, asking which doll she had picked out for her birthday.
"I really lub dis one," Kinley told him.
"They chuckled and he walked off with the doll and said try to stay over here," Shufflebarger wrote in the post on Facebook.
"I thought it was odd at first but was quickly distracted with more oohing and ahhing.
"Kinley asked where the dolly was she picked and I told her it would be okay there are more.
"He came back a few minutes later with her doll, in a bag with a receipt and told her happy birthday, you enjoy your dolly.
"To this generous and humble young man, I hope you see this and more importantly, I hope you know what a good young man you are."
"I was speechless,” Shufflebarger told Fox News. "Seriously. What a reminder to never lose faith in humanity and to be generous and pay it forward."
Since the mum posted the story on Facebook on 23 October, it has been shared more than 70,000 times with hundreds of comments praising the teenager for his good deed.
"This is simply amazing. You're truly a wonderful guy - this made my day to still see great people out there," wrote one person.
Shufflebarger told WTSP the teen's act of kindness has "generated thousands of smiles, softened many hearts and inspired others".
"That in and of itself makes my heart full and happy," she added.
After Shufflebarger's post was shared by thousands of people, someone tagged the teenager - Tario Fuller, a student at Purdue University - in the post.
The University's athletics team then shared the mum's post, adding: "We couldn't be more proud of the type of young men and women in our athletics department."