A 51-year-old man from Blackpool was arrested on suspicion of rape after an alleged attack in woodland close to Stanley Park.
Police were called shortly after 5pm yesterday to a report a 59-year-old woman had been sexually assaulted in a wooded area close to Woodside Drive, off East Park Drive.
In a statement, Lancashire Constabulary said: "Enquiries are ongoing but police believe the victim and the arrested man are previously known to each other."
Officers are appealing for a female dog walker, who raised the alarm but left before patrols arrived, to come forward, along with anybody else with information.
Detective Inspector Alisa Wilson said: "Firstly we would like to reassure members of the public that we believe this was an isolated incident as the victim and the alleged offender are thought to be known to each other.
“However, our enquiries are ongoing and we would now like to speak to anybody with information about this incident, and in particular would like to trace the woman who first called the incident in to us as she may have vital information which could help with our investigation.
“If you know who this woman is, or indeed, if you are the lady in question, we would ask that you contact us as soon as possible.”
Details can be given to police by calling 101, quoting log number 1102 of July 6.