John Edwards said his beloved Gizmo had been missing for two weeks when he learned he had been mistaken for a stray and picked up by the animal charity.
The white puss, described as a “fluffy furball”, was put down the same day.
Mr Edwards, of Gloucester Street, Brighton, said: “I am absolutely seething. He always used to go out walking and all the neighbours knew this.
“My other cat, who is a Siamese, is grieving as he knows his mate has gone.
“My blood is boiling and I don’t know when I’ll calm down.”
Gizmo was taken two weeks ago by animal inspectors from the RSPCA but Mr Edwards only found out his cat had been picked up on Monday after leaflets were delivered by concerned residents.
A spokeswoman for RSPCA said they were called to the street by members of the public who said they thought the animal had been involved in a fight.
She added: “Unfortunately the animal was in very bad health, with bad teeth and matted fur. He was covered in fight wounds when concerned residents asked us for help. we were unable to trace his owner.
“The RSPCA does everything it can to avoid having to put animals to sleep, but sometimes we have little choice.”
"I stopped supporting the rspca many years ago because they are too quick to put down animals.all they are interested in is raising money for political aims.and recently even going to court to try to overturn a request of money that was going to be given to them and then changed.the bill for the court fight was a quarter of a million pounds and that is where your donations are going.i give my money to a local animal charity that truly cares for animals.its about time the queen took away the royal part of this so called caring association."
"Quite agree with you, Jonny. I suffered at the hands of the RSPCA through an interfering neighbour reporting me without understanding anything about my little cat. I rescued him at 4 weeks old with his mum and they were both very ill. She was only 8 when she died, but her little son lived for 12 years. I'm sure that many of us and, more importantly, our little pets, suffer at the hands of the RSPCA, but, hopefully not to the extent of them killing our pets without cause.
My thoughts are with you for your sad loss."
"The RSPCA do kill healthy animals all the time. Even dogs who have been in kennels 'too long', though they won't specify what 'too long' is. Regardless of age or health, they kill one in every three dogs they get, and even more cats!
They even have a noise counter and take out the noisiest dogs and kill them. The RSPCA is not the benevolent Society it wants the public to believe it is. It's the richest animal charity in Britain, it kills the most animals, and all the money goes to HQ, not to its shelters. They have to raise their own funds and pay HQ to use the rspca logo."
"This is typical RSPCA. I completely agree with Queenie. They are too ready to pts animals that are not seriously ill upon admitting them, without even waiting for the owner to appear. Any animal being admitted by the RSPCA could very well be dead by the next day, that is unless there is a TV crew filming. A multi million charity, which must have the slogan `charity begins at HQ´.
This is not the first time a cat not really missing has been taken off the street by them and instantly slaughtered."