The man indecently exposed himself to a 39-year-old woman as she walked towards a pathway that leads to Hylton Road, in Petersfield.
As she reached the pathway she noticed a man exposing himself and committing an indecent act.
The incident happened at about 3pm on Monday.
PC Neil Croutear said: 'Did you see anything suspicious? If you did, please contact us as soon as possible.'
The man was described as white, around 40 years old, of thin build, with dark brown, wavy, shoulder length hair and of scruffy appearance.
Anyone with information should contact PC Croutear at Alton police station on 101 or call Crimestoppers anonymously on 0800 555 111.
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