Three men are being sought by police following the sexual assault which happened on the canal towpath of the Calder Navigation.
The victim was approached by her attackers as she walked along a section of the towpath between the Cromwell Bottom Nature Reserve and the Colliers.
Police said the woman was assaulted and then sexually assaulted by the men, leaving her with injuries that needed hospital treatment.
West Yorkshire Police said the crime had been initially reported to them as an assault, but that it was now being classed as a serious sexual assault.
DCI Sue Jenkinson, of the force's Homicide and Major Enquiry Team, said: “We are investigating what is clearly a very serious offence which has come to light as the victim has gained in confidence and now fully disclosed what took place.
“She continues to be supported by officers and we are carrying out a number of enquiries into what took place.
“I would like to speak with anyone who can assist the investigation or who may have seen or have information about three men who we believe were on the towpath at around the time of the attack.
The attack took place some time between 6.30am and 7.50am on Monday, October 19 and police are particularly keen to speak to three men who may have been together on the towpath at the time.
Anyone who has information is asked to contact the Homicide and Major Enquiry Team on 101 referencing crime number 13180545115, or by calling Crimestoppers on 0800 555 111.