The victim, aged in her 30s, was making her way along the path at the north side of the canal in Knottingley at around 10.30am on Sunday morning.
She was passing under a bridge at Hill Top and Bridge Lane, when a man approached her and sexually assaulted her, before making off from the scene.
A woman walking her dog then appeared on the towpath nearby.
The suspect was in his 30s, around 6ft tall, of muscular build with broad shoulders.
He had light brown hair with white highlights, tanned or olive skin and he spoke with a foreign accent.
He wore a long sleeved grey top and dark blue tracksuit bottoms.
The woman who may have witnessed the assault is believed to have been in her 50s and was walking a white and grey Jack Russell-type dog.
Sergeant Phil Davis, of Wakefield CID, said: “I would appeal for anyone who recognises the man described or who believes they may have seen a man matching this description in the area to please to get in touch with police.
“I would also ask for anyone who believes they know the woman walking her dog to contact police, as she may have information valuable to the investigation.”
Anyone with information should contact Wakefield CID on 101, quoting crime reference 13170243260.
Information can also be passed to independent charity Crimestoppers on 0800 555 111.