Alan Tomlinson believes he saw a ‘panther’ near to junction one of the M180, close to Doncaster services just after midnight on January 4.
The 52-year-old transports medical records between hospitals for emergency patients, and was on his way back from Scunthorpe General Hospital when he saw the creature.
Mr Tomlinson, of Goole, says he realised the animal was a panther when he saw the length of its tail, which he describes as being two to three feet long.
He said: “I’ve been doing this job for eight years, and have been driving since 1982 and I’ve seen foxes and things like that but I've never seen anything like that before. It was much, much bigger than any animal I've ever driven past.
“I only saw the back of it,but I knew it was a puma or a panther when I saw the size of its tale which was curled right round. That's how I knew it was a panther.
"After Googling it and seeing pictures and videos of panthers I’m even more sure now.
“It was walking along the road slowly and really casually, as if it didn’t have a care in the world. I was gobsmacked.
“It crept under the barrier near to the side of the motorway and then walked off into the grass verge.
“I’ve told the police and wanted people to know about it, because I wouldn’t want to not say anything and then find out someone had been attacked by this panther.”
Big cat researcher Paul Westwood monitors sightings of panthers and pumas across Britain and says he urges anyone who sees one to contact the police.
He added: “Sightings of panthers have gone down recently, I haven't been contacted about any in a while, but it is possible that one has been seen in Doncaster.
"I wouldn’t want to rule it out. It may also be that he saw a domestic cat, but it's hard to say for sure.
“There’s never been a reported case of a big cat biting anyone in this country, but I would still recommend that people don’t keep sightings to themselves and contact either myself or their local police force."
South Yorkshire Police have been contacted for comment.