Officers want to speak to anyone who has information about the incident which took place between 3.20pm and 3.30pm in Crow Nest Park, Heckmondwike Road, on Friday February 15.
The suspect approached the girl from behind, attracted her attention and then grabbed her in a bear hug as she turned to walk away.
The man then sexually assaulted the victim, who was struggling, before running off.
He is described as Asian, of medium build, 5ft 10in, with dark wavy hair cut short with a short fringe and in his late 20s or early 30s.
The suspect was wearing a light blue traditional Asian style dress with matching trousers underneath and a black quilted jacket.
He spoke with a heavy accent and was mumbling and slurring his words.
PC Iain Adair of the Dewsbury Neighbourhood Policing Team, said: “We are investigating this matter and I would like to speak to anyone who has information or who recognises the man pictured to contact the Dewsbury and Mirfield NPT on 101.
“Information can also be given anonymously to CrimeStoppers on 0800 555 111.”