The victim was in Church Lane, Northampton, walking towards Campbell Square, sometime between 8.30pm and 8.50pm on Monday, 21 March.
The man was stood in the gateway to the Holy Sepulchre Churchyard when he exposed himself and shouted something at the woman.
The victim backed away into the road and tried to flag down a motorist, believed to be a woman wearing glasses and in her forties.
The man is described as white, approximately 35 to 40 years of age, 5ft 8ins to 5ft 9ins in height, of stocky build, clean shaven and with a clear complexion.
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He was wearing a black woolly hat turned up at the bottom, a black bomber jacket and grey tracksuit bottoms.
Officers are appealing for the motorist or anyone with information can contact Northamptonshire police on 101. Alternatively, they can call the independent charity Crimestoppers anonymously on 0800 555 111.