Tayla Smith, from Swindon, saw white Staffordshire Bull Terrier 'Molly' for sale on an Internet selling site in April last year and instantly fell in love.
Tayla was told by the seller that Molly was a loved family dog, was only 11 months old, and was not microchipped.
She said: "We bought her with completely good intentions. My partner and I are both self employed and we changed our lifestyles to fit around Molly.
"I was so desperate to have her, I fell right into the trap."
She collected Molly on April 23, loving and caring for her new pet - completely unaware that her owner was desperately missing and searching for her.
It wasn't until December, Molly injured her claw and had to be taken to the vets - requesting Molly be microchipped.
It was then, the vet told the couple that Molly was in fact already microchipped and after looking into the details, the vet confirmed that Molly had been reported stolen.
In fact, she wasn't called Molly at all. Her real name was Storm.
Tayla said: "I was so confused, I didn't know where to start. I just wanted to speak with the owner.
"It's bittersweet. It's something you hear happens, but you never expect it to happen to you."
Tayla contacted the microchipping company and the police and was eventually given the details of the owner - Dean Allen, from Wakefield.
"When I spoke to Dean, he just crumbled," Tayla said. "I have never heard someone sob so much."
They arranged to meet with Dean to reunite him with his beloved Storm.
Tayla said: "It's heartbreaking but at the same time we're happy. We thought she was only a puppy, but it turns out she's six years old!
"Molly had such an impact on our lives, but I'm just so happy she's now back where she belongs.
"If we had just checked earlier for a microchip, we probably wouldn't have grown so attached to her, but it's better late than never."
Dean said: "I am extremely emotional. I'm also devastated for Tayla as she loved her so much.
"I'm just so glad that Tayla went through all the effort to give Storm so much love. I couldn't have wished for my worst nightmare to become a dream and I couldn't have dreamt for a happier ending.
"To me, Tayla is family now. She loved my dog so much. I just can't praise her enough."