A flasher confronted a 16-year-old girl as she walked along a path in Duffield.
The man dropped his trousers and was seen performing a sexual act as the girl made her way along a footpath near Ecclesbourne Meadows.
The teenager continued walking towards Holloway Road, while the man ran off towards the River Ecclesbourne.
She then saw the man again on Holloway Road, and walked past him.
The man who exposed himself is described as being in his late twenties and was wearing a scruffy grey t-shirt and baggy black jeans.
The incident took place at around 6.20pm on Friday, June 7.
A spokeswoman for Derbyshire Police said reported cases of flashing were rare in Duffield.
She said: “It’s a very unusual incident to take place in that area.
“If anyone has seen anything at any time that they consider suspicious or unusual in that area then come forward and let us know.”
Latest available figures for April showed there was just one violent crime — which includes sexual offences, grievous bodily harm and common assaults — in Duffield during that month.
There were none in either February or March, with one violent crime reported during January.
The Ecclesbourne Meadows area is only a few hundred metres from Ecclesbourne School.
If you saw the incident or have any information then contact Derbyshire Police by phoning 101 or Crimestoppers anonymously on 0800 555 111.