Officers were called at 7pm on October 6, following reports a woman had been attacked on Messingham Fields, off Holme Lane.
She fought back and was able to get away.
He was described wearing dark coloured clothing and a black hooded jacket.
DS John Shepherd said: “Incidents of this nature are rare, but are extremely distressing for the victim and the community as a whole.
“We are working with the victim and we also wish to reassure the public that a thorough investigation is taking place.
“Our request to the community is that anyone who has information in connection with the identity of the man or knows anything relevant to the investigation comes forward to speak to us as soon as possible.
“Regular patrols are being made in the area and, while there is no suggestion the victim was at fault, we would also encourage people to consider their personal safety and be aware of their surroundings.
“Try not to walk alone, especially at night, but if you have to, ensure you keep to well-lit areas, busy streets, main roads and footpaths. Avoid badly lit areas, parks, alleyways and underpasses.”
A 33-year-old Scunthorpe man initially arrested in connection with the incident has been released on police bail after helping police with enquiries.
Anyone with information about the incident is asked to call 101, quoting crime reference number 2142896 of October 6.