The man is reported to have pushed the woman onto the floor on Far Ings Road near Waterside Road before kneeling on her and touching her inappropriately.
He then ran off in the direction of The Sloop Inn pub.
The woman suffered scratches to her chest and face.
The suspect is described as white, medium build, approximately 6ft to 6ft 2ins tall, aged between 30 and 40, with short brown hair.
He was wearing blue denim jeans and a lighter coloured blue cotton zip-up jacket.
The alleged attack happened on March 26 around 4pm.
Anyone who can assist Humberside Police with their enquiries is asked to contact the force on 101 quoting reference number 16/36296/18.