The victim was walking through the park when she was approached by a man who grabbed and indecently touched her.
The girl broke away from the man who left the scene with her phone, which he dropped a short time later.
The victim was left uninjured but understandably shaken by the incident, police said.
DI Julie Deacon of Bradford District Police said: “A substantial number of enquiries have been ongoing into what has been a serious but isolated incident, which we have recorded as a sexual assault.
“We are now in a position to release CCTV images of a man we would like to speak with in connection with the incident, and would ask anyone who can help identify him to get in touch.
“Anyone with information about the man or the offence is asked to contact police on 101, quoting crime reference 13210519025. Information can also be given online at www.westyorkshire.police.uk/101livechat anonymously to the independent charity Crimestoppers on 0800 555 111.”
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