Laura Cunliffe, 23, of Barnsley, put her 4-month-old kitten in the microwave as punishment for attacking her goldfish on September 28, The (Sheffield) Star reported Thursday.
After a minute, Cunliffe "came to her senses," took the cat out, and poured water on it to cool it down, prosecutors said.
The kitten died about 90 minutes after the incident, Royal Society for the Prevention of Cruelty to Animals inspectors said.
The crime came to light when Cunliffe checked into a hospital the next day for unrelated reasons, where she confessed what she had done to a nurse. The nurse reported the incident to the RSPCA.
"You were aware of what you were doing. This was an act of truly horrendous cruelty. The animal suffered great pain and that suffering was entirely foreseeable from the moment you chose to do what you did." District Judge John Foster told Cunliffe.