Hannah Johnston, 29, from Bury St Edmunds, cooked the £5 roast pork loin from Farmfoods, but when she cut through the meat she realised there was something hard inside.
She said: "When it was on the board, I cut down and hit something hard. Then I found the keys.
"I was really shocked. It wasn’t nice to see."
Johnston was cooking the pork to feed to her three-year-old son
The mum said she was also cooking for her two friends, Shanna Tingey, 21, and Shanna’s boyfriend Ben.
She explained: "I bought this one last week and cooked it straight from frozen for an hour and a half.
"I checked on it after 30 minutes and couldn’t see anything wrong, and when it was ready brought it back to the chopping board."
This was the point Johnston cut through the pork and found the keys.
Johnston often buys food from Farmfoods
Johnston immediately contacted Farmfoods and said she is waiting to receive a voucher from the chain supermarket "as a good-will gesture".
The mum-of-one said she was pleased that the frozen food chain were apologetic when she complained.
"They said this has never happened before," she said.
"They want me to send back the keys so they can look into it, but I am not keen as I think it will then go away.
"They are going to send me a voucher too, but I don’t know how much it will be worth."
Johnston said she wanted people to be aware this can happen and to "be careful", adding: "It has put me off shopping there."
Farmfoods said it would not comment on individual incidents.
A spokeswoman said: "If a customer has been in touch with an issue we will make contact with the supplier and investigate."