Police officers are trying to trace a man in his 40s, with a heavy build and a bald or shaved head, who is thought to have carried out the attack.
The woman was in Callender Park View in Falkirk in the early hours of yesterday morning when she was sexually assaulted, police said.
They are carrying out extra patrols in the area and are keen to speak to anyone who was there at the time of the attack, thought to have happened between 12.30 and 2am. Detective Inspector Hugh Louden, of Police Scotland, moved to reassure members of the community.
He said: "This is a very unusual occurrence and I would like to reassure local people that Police Scotland are conducting a thorough investigation and that additional officers are carrying out extra patrols in the area to keep people safe."
The man, who is thought to have been wearing blue jeans and a light-coloured long-sleeved jumper, was last seen in the area of Callender Park View at around 2am.
Mr Louden appealed for anyone with information to come forward to police.