The neglected boy, also suffering from a skin infection and lice infestation, was discovered nearly naked at a mechanic's workshop in Arica, more than 1,600 kilometres north of capital Santiago, police said.
Neighbour Lory Escudero spotted the boy being breastfed by her dog Reina, hailed as a hero by the locals, and called the police last Thursday.
"We have our dog who is there and she's pregnant. The hungry boy suckled on the dog's teats for milk," Escudero said.
The boy was taken to a hospital where he was treated for malnutrition and skin infections before being released on Friday to be taken care by the child welfare authorities.
His mother visited the hospital, but was inebriated, police said, adding that she has not been placed under arrest as there had been no physical harm to the boy.
The incident took place in an impoverished area of Arica where thousands of Peruvians and Bolivians arrive in search of work.
