Authorities in Song Cau Ward said the boy was found at around 9 am on Sunday following an overnight search involving local officials and residents.
The boy had left his home in Long Hai Bac neighbourhood at around 3.30 pm on Saturday and lost contact with his family. His disappearance caused particular concern as he has developmental delays.
He was wearing a dark blue T-shirt, dark blue shorts and blue plastic sandals when he left home, accompanied by the family's black dog.
Authorities and local residents searched several areas throughout the night before finding the boy in mountainous terrain more than 10 kilometres from his home. He was in stable condition.
The dog, which had followed him from home, was still by his side when rescuers arrived after staying with him throughout the night.
The family said they had raised the dog for about two years and that it was particularly close to the boy, often accompanying him in his daily activities.
After being given food and water, the boy was safely taken home.