Dog walker Andrea Critchley shared the alert on Facebook after spotting the half-naked man near Sprotbrough Falls yesterday afternoon.
In the post, which has been shared nearly 300 times on social media, she wrote: "WARNING !! Went for a walk down to Sprotbrough Falls this afternoon with my dog .. went about 2 miles down under the motorway bridge.
"There was no one in sight when I heard a whistle - looked across canal and there was a bloke pants down, t****r in his hands shouting 'I'm coming over and your getting some.
"I absolutely s*** me sen ! So I just ran fast as I could back to the Boat Inn.
"Be careful re your kids and not to go on your own."
The Don Gorge area is a popular spot for walkers, cyclists, runners and anglers while the nearby Boat Inn at Sprotbrough is a popular stopping off point for canal users as well as hikers.