Beki O'Rourke, 28, from Dundee was enjoying the snack before heading out for the afternoon with her three-year-old son and partner Mike, 30.
Biting down into the pasty, Beki said she felt 'something hard' between her teeth. Closer examination revealed a 10 pence piece hidden among the layers of the pastry.
The outraged mum told the Dundee Courier that when she phoned the bakers to complain the manager laughed at her and said it was a 'random' occurrence.
"I got about three quarters of the way through it when I felt something hard. I thought it was a bit of uncooked pastry, but when I opened it up I found the 10p between the layers," she said.
"I was disgusted - money is disgusting because so many people handle it and you wouldn't want it in your mouth. The annoying thing though was when I phoned and asked for the manager I got this guy who laughed when I told him."
She said she told the shop keeper that it was 'disgusting and unacceptable' and that she wanted to make sure it did not happen to anyone else.
"He said if he had seen it he would have taken it out before the bake was sold. It's unbelievable. He said he was the sales manager running the store that day but he didn't ask me any details about it. He didn't seem concerned at all," she said.
Beki reported the incident to Greggs via their website, and also to the environmental health department at Dundee City Council, who came and picked up the remains of the bake from her.
A spokesman for Greggs said they had launched an investigation and will be in direct contact with Beki.