The woman, who did not want to give her name, said she saw the man performing a sexual act in Endcliffe Park at around 9.15am on Thursday as she walked her border collie puppy.
The man was clothed but exposed himself just off a path in a wooded area that leads to Notre Dame High School.
“It was dirty, unpleasant, and all-round distressing,” the woman said. “I was across the river from him, and he stared right at me as he did it.
“The worst thing is my own children and other kids use the path every morning on their way to school, and if he’d been there just 10 or 15 minutes earlier, they could’ve seen what I did.
“I was cross with myself afterwards as I didn’t shout at him. If I did, other people would’ve seen, and it would’ve been far more embarrassing for him. You just don’t think about those things when you’re in a state of shock.”
The incident has since been reported to South Yorkshire Police and is under investigation but no arrests have been made so far.
Other people have since reported similar experiences at Endcliffe Park after the woman posted about the incident in a local community Facebook group.
Several people have also speculated whether the the same culprit is responsible.
One woman said: “I had a similar experience in that location about a year ago. It’s very worrying.
“It made me wary of walking my dog along that path and I am certain I saw the same man acting strangely there a few months later.”
And another added: “This happened to me and my friends last year in Chelsea Park, could be the same man. He appeared just after a group of children had been playing. The police are aware of it.”
On Saturday another woman urged others to be “safe and extra vigilant” after claiming “an older male” “masturbated at me” in Chelsea Park.
Police were called and searched the area.
The woman said: “Really concerning as there’s lot of children in this park.
"He was of average height, medium build and had brown hair.”
The Star has contacted South Yorkshire Police for comment.