Mum Paula Williams posted the video of her son to the Lost and Found Pets of Texas Facebook page in a bid to "share some good news".
"My son's puppy went missing about a month ago and we just found him," she wrote. "This is the video of him reuniting with his baby Kase. He was found right down the street from our house.
"A nice family had taken him in and kept him warm and fed."
The video shows Williams' son walking up to the van where his puppy is being kept, with a confused look on his face.
When he sees his puppy staring back at him in the van, he looks stunned.
The boy immediately wraps his arms round his puppy and breaks down in tears, telling him how much he loves him.
The video had been shared more than 60,000 times within a week of being uploaded on 8 January, with nearly 8,000 comments.
It prompted other parents to share photos of their children and pets' "unbreakable bond", and moved many Facebook users to tears.
"This is beautiful thank you so much for sharing, I'm in tears," wrote one person in the comments.
Another person added: "Wow I'm sobbing. Your son is precious and showed his love so openly."
The video has been viewed nearly four million times.