The dog, called Eragon, was found by children attending a service at St Mary's Church, Hitchin, on Tuesday.
He was accompanied by a note asking, "please find me a lovely new home".
The RSPCA scanned his microchip and found he belonged to Samantha Jones from Bowditch in south London. She had lost the dog during a walk in August.
The reunion was "a lovely surprise", said Ms Jones, adding: "I thought I would never see him again.
"I was especially shocked to find he was in Hertfordshire - quite a way from my London home."
The three-year-old animal was discovered tied up in a Santa sack with food, treats, a jumper and a Christmas card.
The note said: "My lady is going into a home. I need food cut small. Please find me a lovely new home. So sorry."
The RSPCA believes Eragon was found by someone who took him on without thinking to check his microchip and then found they could not cope.
RSPCA inspector Stephen Reeves said: "Eragon had clearly been cared for, as the treats, jumper and card show, but it it could have ended quite badly to leave a little dog like that out with no care in the temperatures we've been seeing recently.
"It was certainly one of the more unusual and most festive rescues I have ever been called out to."
A Chihuahua dumped at a church in Hertfordshire in a Santa sack has been reunited with his owner four months after he went missing.