Nicholas Murphy, 22, took his Jack Russell, Gunner, to the Worcester-based DJB Denny veterinary practice after a persistent cough would not shift.
When medics gave the dog an injection without warning in front of Mr Murphy's 14-year-old brother, he simply assumed it was antibiotics.
Mr Murphy, who lives with his mum Karen Wood, 48, sister Hannah, 17, and brother Paul Wood, 14, said: "He asked us if Gunner had had enough, I just thought he meant of the cough, then he gave him an injection.
"He never asked us anything about the injection, he just put the needle in Gunner."
But he only found out it was in fact a lethal cocktail of drugs to put the 14-year-old dog to sleep when a vet asked if they wanted to bury him at home or at the surgery.
Mr Murphy, an animal welfare worker, described how the mistake left the family "shellshocked" and "confused".
He said: "We just want him back.
"My dog had a bad cough and the vet gave him tablets to control it. But it wasn't working so on Wednesday I took him back.
"I was with him and the vet gave him an injection. I thought it was antibiotics. But it was to put him down.
"There was no consent form, and no permission.
"He said to me 'do you want to bury him at home or here?' and I was like 'what are you on about?'.
"All he said was he was very sorry. He admitted he was wrong to me and said he was very sorry but mistakes do happen.
"I was shellshocked, I just didn't know what to do, I was confused as to what was going on. We had to bring him home and bury him.
"It's been bad since he has not been here. We usually see him running around and coming into the bed.
"Now it just seems quiet, we have had him since he was born and we still have his dad.
"Gunner didn't deserve to be treated like that. All he needed was some stronger tablets for his cough."
He said that he has made an official complaint to the Royal College of Veterinary Surgeons.
Ian Holloway, a spokesman for the organisation that monitors veterinary practices in the UK said: "Whilst we are unable to comment on whether or not a complaint has been made against an individual veterinary surgeon, unless and until it might be referred for a full Disciplinary Committee Inquiry, we do investigate all complaints we receive to determine whether there are any potential issues of professional misconduct or questions over a veterinary surgeon's fitness to practise.
"Full details of our complaints investigation process are available on our website."
Mr Denny said he did not wish to comment on the incident.