The incredible discovery was made at Norala, in the South Cotabato province of the Philippines, on June 29.
The abandoned infant was taken in by local social services as the search for his parents continued.
After being found he was initially taken to hospital to be checked over.
Despite his crying, health checks revealed he was okay apart from being slightly dehydrated.
Mary Kris Dela Cruz said her uncle’s workers heard the faint cries which led them to the blue backpack hanging on the fence outside their house.
She said: “It was early in the morning when we heard the baby crying.
“It was strange because we had no baby in the house. We searched where the sound was coming from and it led us to the front gate.
“Something was moving inside the bag and it was the baby.”
A spokesman from the Provincial Department of Social Welfare and Development said in a statement: “The baby will be under our care even after the parents were found.
“We just want to remind everyone that struggling parents could always ask us for help if they don’t know what to do to their babies instead of leaving them alone and abandoning them.”