The cat was found in the microwave in Rochester on Saturday after its owner heard it being turned on.
The animal survived but may have suffered internal damage, the Royal Society for the Prevention of Cruelty to Animals (RSPCA) said.
The youth offending team at Medway Police Station and social services had been contacted but the boy was too young to face criminal charges, the BBC reported.
"This was an absolutely horrendous incident and all the more shocking because of the alleged involvement of such a young child," it quoted RSPCA inspector Ray Bailey as saying.
