Ronnie and his mum Michelle Humber had gone to pick a replacement pet when they stumbled across their old cat Phoenix who vanished from their home in May 2014.
Ten-year-old Phoenix was in an enclosure next to the cat Orlando who they had arranged to collect from their local Cats Protection charity.
The family now plan to give both cats a home.
Mum-of-two Humber, 33, said: "It was amazing and the children were so pleased. Ronnie saw him straight away and was screaming and cheering with excitement.
"He was purring as soon as we went over to him and it was amazing to be able to hold him again."
Humber said her two boys were devastated when Phoenix went missing because he played such a big part of their lives.
Phoenix disappeared from the family's home in Littleover, Derbyshire last year, but despite plastering the neighbourhood with posters, he never turned up.
"When he went missing, we'd no idea where he'd gone," Humber said.
"We got told he had been taken in by an old man but we never saw him and we eventually just gave up hope of finding him.
"It came to the point a few weeks ago where we were talking about getting a kitten. The lads were really keen on the idea because they missed Phoenix so much."
The family eventually gave up hope of finding Phoenix, who they had got as a three-year-old in 2008, and continued to look after their other moggie Ditto.
But when Ditto died last month, the family decided to go to the Cats Protection centre in Derby to pick out a new cat.
"We saw Orlando and went back a week later to pick him up and that is when we spotted Phoenix in the next door enclosure," said Humber.
Humber said Phoenix was still registered with his microchip as living at their old address and he had a little wonky ear, so all of that confirmed it was him.
Staff at Cats Protection told the family Phoenix had been brought in to the charity two months ago after someone took him to the vets as a stray.
Humber's partner Jay Cockroft, 38, a binman, added: "It's a bizarre coincidence but we'll all delighted to have Phoenix back. The boys couldn't believe it when they saw him."
Humber added: "I know the lads are buzzing with excitement because that's all they're talking about.
"He's a really friendly and pleasant cat. He's perfect and great with the children.
"The staff were brilliant. I'm not sure if they've ever seen something like this happen before.
"They're very professional and because they've cared for Phoenix so well it means we can welcome him back into our lives.
"I don't know what the odds are of what happened to us but I imagine they're pretty small."