The woman, who did not want to be identified, said she was walking her dog at around 2pm in Norfolk Park, Sheffield, on Friday, November 19, when she saw the man performing a sexual act.
The man, who she said was in his late 40s or early 50s, was seen near the trees just by the basketball courts.
She said: "At first I didn't know what he was doing, and when I looked up, my first thought was why is he naked?
Then it hit me. I was like 'oh my god’, and I completely froze and took my phone out. I shouted 'what are you doing?'.
"Then he went back into the bushes and I did not see anyone come out, so I phoned the police."
‘I no longer want to go there alone’
She said that in her conversation with the police she was informed there had been five instances of a similar incident in the park, but no one had been caught.
The woman, who goes to the park every day, has stated that she will no longer go there alone.
She said: "I don't want to walk there on my own. I usually walk where he was, and now there is no way. I'd just go straight home.
"It just makes you think, like, what is wrong with these people? It's sexual assault, isn't it?
"It's shocking. I was just really p****d off that this kind of stuff happens. This was my first encounter and I was just so shocked."
She added that the location was not covered by CCTV.
"I spoke to the park ranger and the CCTV they’ve got only covers a main community centre building and it doesn’t cover any parts of the park.
"There are loads of different exits. They could have literally gone anywhere.”
Similar incidents had happened in other Sheffield parks
Several women had reported similar incidents in a number of Sheffield parks previously.
In July, police were notified that a male had indecently exposed himself in Chelsea Park, Nether Edge, according to witnesses.
In a different incident that occurred earlier in the month, a man was caught by dog walkers performing a sexual act in Endcliffe Park.
The man had been exposing himself just off a path in a wooded area that leads to Notre Dame High School.
Both incidents happened in broad daylight and it is not known whether the suspects are still at large.
South Yorkshire Police, when contacted, confirmed receiving a report about the Friday incident and are now urging those with information to come forward.
A spokesperson for the force said: “We were called at about 2.05pm about reports of a man performing a lewd act in a Sheffield park.
“The incident reportedly occurred near the basketball courts in Norfolk Heritage Park on Park Grange Road.
“The man was described as white and possibly aged in his 40s.”
If anyone was in Norfolk Heritage Park and saw or heard anything suspicious around this time they are asked to call 101, quoting incident 456 of November 19.