The incident took place near a public telephone box on Henderson Place, Keyham between 2200 and 2230 hours on Tuesday, 16 June 2009.
The girl was walking home with her friend, also 15-years-old, when the man exposed himself.
The man is described as:
- being around 30-years-old
- 5ft 6in tall
- of chubby build
- with short dark hair
- wearing dark clothing
Anyone with information is asked to contact the Police on 08452 777 444.