Sussex Police said officers were called at 12.25am on Saturday (June 13) after the 25-year-old victim was attacked as she was walking on a footpath through Whitebeam Woods in Durrington.
The attacker forced the woman to the ground and sexually assaulted her, said police.
The attack happened on a path that leads from the Yeoman car dealership on the A259 to Whitebeam Road. The woman was sexually assaulted while she was close to the basketball court that is opposite the junction of Romany Road and Walnut Tree Way.
Crime scene investigators are currently collecting evidence from the scene and officers are carrying out house to house inquiries and searching the area.
Detective Chief Inspector Paul Rymarz, from Sussex and Surrey major crime team, said: “The victim is understandably upset by what has happened to her and specialist officers are supporting her.
“We need the help of the local community to identify and trace the person involved.
“The attack happened just a short distance from a number of homes so I am keen to speak to anyone who saw or heard anything suspicious in or around the woods or the paths that lead out of the woods late on Friday night or just after midnight today.
“We would also like to speak to any motorists who used Yeoman Road, Romany Road or the A259 Littlehampton Road who may have seen someone hanging around or running on a footpath out of the woods between 11.10pm yesterday and 12.25am today.
“Crimes like this are extremely rare in the area and we are not currently linking this to any other incidents.
“If you know anything at all, please contact us.”
Anyone with information should email [email protected] quoting Operation Mode, call 101 or contact the independent charity Crimestoppers anonymously on 0800 555111.