Shocked Cheryl Jaconelli, 23, was left feeling sick after the gruesome discovery as she tucked into a six-piece variety bucket.
The student nurse and boyfriend Christo-pher Neilly, 22, bought the food at the chain's Parkhead Forge restaurant on Tuesday.
Cheryl, of Glasgow, took a picture and uploaded it to Facebook, where, within hours, it had been viewed by more than 3000 people.
Bosses have launched a probe after she reported the discovery.
Cheryl said: "Christo-pher and I had been to the drive-through at about 11pm. I was on my last piece of chicken when I noticed it didn't look right. My first thought was it was a chicken's brain. I let out a scream because of the shock. I instantly felt sick. It was horrible.
"I called KFC to complain and the girl I spoke to was very apologetic. She told me to fill out an invest-igation form and send away what I'd found."
KFC apologised and claimed that Cheryl had discovered a piece of kidney and not brain.
A spokeswoman said: "We are really sorry for Miss Jaconelli's experience. We sell 200 million pieces of chicken every year and always try to ensure the highest standards.
"However because all our chicken on the bone is freshly prepared by hand, unfortunately on extremely rare occas-ions, human error can mean that a kidney, and not a brain as claimed, is not removed in the preparation process."