Police said the 20-year-old woman was on a footpath that leads on to Northampton Racecourse on Wednesday night when she felt a man touch her.
He forced her into the town centre park where he raped her, police added.
The man was white, aged in his early 20s, about 5ft 10in, of medium build and he had short dark hair and stubble.
Distinctive clothing
He was wearing a mustard-coloured zip-up cardigan that had the number 78 stitched on the left-hand side of the chest, dark jeans and open, walking-style sandals.
Det Ch Insp John Jones said he believed many people might have noticed the man's distinctive clothing.
"The woman has undergone an awful experience that has been very traumatic for her and we want to find the man who attacked her as soon as possible."
Before the attack, the woman had got off a bus at Greyfriars bus station and walked down Campbell Street to Regent Square and along Barrack Road.
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