Tom Pitt, 38, was walking his dog in Hatfield Park, located on the border between Hertfordshire and Essex, when he saw the dangerous animal in the distance.
Mr. Pitt, who lives in Hatfield, initially thought he was a big domestic cat before approaching him, only to have the rhythm growl and show his teeth.
Mr. Pitt said: "I was walking my dog there, which is what I usually do.
"It was near the Essendon end of the park, which is much quieter than the rest of the park."
He added: "I think it was my dog who noticed first, looked in that direction and I looked, and it was a big black cat, it looked like a domestic cat but much bigger."
Miraculously, Mr. Pitt managed to take some pictures of the animal before fleeing with his dog.
The images show the great black cat, which the local media has described as a panther.
Mr. Pitt said: "We made eye contact with him and started walking towards him.
"He seemed to grunt and show his teeth.
"He ran into the forest after that. I think we startled him, everything was very fast. "
Mr. Pitt said the sighting made him understandably worried.
He said: "I've been walking around there for years and I've never seen anything like it before.
"You do not expect to see that kind of thing.
"It was morning and I was barely awake. You see deer and hedgehogs, but not a big cat. "
Mr. Pitt's photos show a large black animal lurking in a field.
Although strangely enough, the RSPCA confirmed that there have been no reported sightings that match the description in the park.
Mr. Pitt's encounter comes two days after Scottish police issued a warning to residents after a "panther" was seen loitering in Ayrshire.
They scoffed when an image of the animal revealed that it was a black domestic cat.
Although the resident Robbie Brown took the opportunity to recreate the alert by sharing images of a cat figure cut on Facebook, which generated fear among thousands of Scots.
Since then, he has confirmed that he did not make the original report to the Scottish Police.