Officers have launched a county-wide appeal to find the attacker following the assault.
The incident took place in Petersfield between 12.45am and 1.05am on Monday, May 16.
A 25-year-old woman was sat on a concrete bridge in Goodyear Meadow listening to music when an unknown man approached her from behind.
The man then pulled her back from the bridge and sexually assaulted her before she managed to run away, police said.
The attacker is described as black, aged between 25 and 30, 5ft 11in to 6ft tall and of stocky build.
He was wearing a black puffer jacket with a hood, black Nike tracksuit bottoms, black leather/ faux leather gloves and black trainers.
Detective Constable Jonathan Duggan from Basingstoke CID said the force was looking to speak to anyone in the area who may have seen the suspect or who was driving and may have recorded valuable evidence on a dash-cam
‘Whilst we are treating this as an isolated incident, we would like to reassure the local community that we are carrying out a thorough investigation and all lines of enquiry are being followed,’ DC Duggan added.
‘We are really keen to hear from anyone with information that may assist us in identifying the man responsible.
‘If you have any information that could help our investigation, no matter how small or insignificant it may seem, it is vital you get in touch.’
People with information can call 101 quoting reference 44220192334, or ring Crimestoppers, anonymously, on 0800 555 111.