The 43-year-old victim was walking through Lovell Park near Leeds city centre when the man exposed himself to her.
He then ran off across Grafton Street and up an alley towards Lovell Park Hill.
The incident happened at 6.45pm on Wednesday November 30, but police only released details today.
He was described as white, about 5ft 6in to 5ft 7in tall, aged 45 to 50, medium to big build, with short black or dark brown hair with grey mixed in.
He was wearing black tracksuit bottoms, a black jacket and black shoes.
Anyone who recognises the man in the image or who has any information that could assist the investigation is asked to contact PC 1399 Kieran Rochell at Woodhouse Lane Police Station via 101 quoting crime number 13160714092 or call the independent charity Crimestoppers anonymously on 0800 555 111.