Beth Waller, 18, made the gruesome discovery just seconds after buying the McChicken Sandwich meal at the fast food giant's branch in St Sepulchre Gate.
And she says that since the incident she has refused to eat chicken - and will never eat at the chain's restaurants again.
Beth, from Dunsville, had popped into the branch with her boyfriend Ben Appleby, 20, for a bite to eat while on a town centre shopping trip.
She said: "I had only had a couple of bites and it didn't taste right. There was a funny taste so I knew something wasn't right.
"When I had a look I could see what looks to be a dead baby mouse inside the burger. I didn't pull it apart - just the sight of it made me feel sick."
A photo of the burger appears to show tiny paws and tail poking out from inside the meal.
She said: "I'm really shy and I didn't report it and take it back. We just got up and walked out, we were that shocked."
But since sharing her photos on videos on Facebook, the meal horror has gone viral and she says McDonald's have now been in touch with her and have promised to investigate.
"I didn't say anything at the time," she said, "but they have contacted me. The woman said it looks like a chicken vein or something but I don't think it is - it looks like a dead baby mouse to me."
She said that the firm had told her it would carry out an investigation.
Added Beth: "I won't go to McDonald's again. I've not touched chicken since and it has put me right off."
A spokesman for McDonald's said: "Food safety is our highest priority and both our restaurants and our suppliers place great emphasis on quality control and follow rigorous standards in order to avoid imperfections in our products.
"On this occasion, we did not meet those standards and apologise for that.
"Unfortunately, the customer did not make us aware of this issue with their food at the time, as ordinarily we would ask the food to be returned to us in order to investigate.
"Looking at the video provided, we believe the item in question was a blood vessel, which naturally occurs within the chicken breast fillet. We have attempted to make contact with the customer a number of times and would like the opportunity to speak with them to discuss the issue.”