The 37-year-old victim was attacked by a man while she was out walking on Upper Common Lane in Clayton West, near Wakefield’s Yorkshire Sculpture Park, at about 1pm yesterday.
After the attacker made off, the woman was found and helped by a man and a woman in a car.
Det Insp Seth Robinson, from West Yorkshire Police’s Kirklees Safeguarding Unit, said: “I would appeal directly for any witnesses to this incident to come forward and particularly want to speak to the two people who came to the victim’s assistance.
“This was a particularly nasty atack and the victim is being supported by specially trained officers.”
Anyone with information is asked to ring West Yorkshire Police on 101, quoting log reference 1092 of March 21.