The man, described as a "Good Samaritan," has come forward following an appeal by officers in Chorley.
DC Debs Parkinson, of Chorley CID, said: "We have now located the man who came to the aid of the victim following the attack, and he has given us further information to help with our investigation."
The attack happened at around 9:16pm on Sunday October 24 in an alleyway at the back of Douglas Close, off Hodder Avenue.
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The man stepped in to help the victim and tried to help her as she was in distress, before alerting two members of the public who subsequently called the police.
Officers say he has now been interviewed by detectives and they have renewed their appeal for anyone else who might have information to come forward.
"We are continuing to appeal for information about the incident itself and are asking anybody who saw it, or has any other information about it, to get in touch straight away," said a spokesman.
The suspect is described as being approximately 5ft 5ins, aged between 40 and 50, with greying dark hair and dark/grey stubble. He was wearing dark jeans, possibly navy and a dark top.
DC Parkinson, added: “Firstly I’d like to thank the people who shared our last appeal.
“We understand residents remain concerned by this incident and we’d like to reassure them that a dedicated team of officers has been assigned to this case and they are working hard to establish who is responsible. Reassurance patrols have also been increased in the area as a precaution.
“We are continuing to ask anybody to come forward if they saw or heard anything in the area around the time of the incident or saw anything suspicious.
"We would also ask residents around the Hodder Avenue and Douglas Close area to check if they have doorbell, CCTV or dashcam footage which shows anything of significance.”
Anyone with information is asked to email [email protected] or call 101, quoting log 1516 of October 24, 2021.