Heather Clark lost her 7-month-old son, Lukas, three years ago after he was physically attacked by his babysitter's boyfriend.
But on Jan 29 this year, Clark heard Lukas' heart beating again - through another child's chest.
Clark had donated her son's organs after his death, and his heart went to Jordan Drake, a 4-year-old girl who was born with a congenital heart defect, according to The Washington Post.
Drake spent the first few years of her life in a hospital, CNN reported, and doctors said her only hope was a heart transplant.
A video of Clark's emotional meeting with Drake and her mother, Esther Gonzalez, has since been shared widely online.
It was their first time seeing each other since the organ donation in 2013, The Washington Post reported.
According to online news outlet Inquisitr, the meeting was held at Phoenix Children's Hospital in Arizona with the help of Donor Network of Arizona, a non-profit organisation.
In the video, Clark can be seen listening to her late son's heartbeat while holding a stethoscope held against Drake's chest.
"That's your baby," Gonzalez said.
A visibly emotional Clark replied: "It's so strong."
Before Clark's meeting with Gonzalez and her daughter, she had told CNN that she was "excited" to get to hear her son's heartbeat again.
During their meeting, Drake also presented Clark with a teddy bear that produces the sound of a heartbeat when pressed.
Gonzalez told Clark that the sound was actually Lukas' heartbeart.
"It's perfect," Clark said.
Lukas' alleged attacker, Ryan Anthony Garcia, is currently on trial, according to a Facebook page called Justice For Lukas Scot Clark.