The girl was walking through Mayfield Park when she noticed a man following her. When he caught up to her, he put his hand up her skirt.
When the girl screamed, he fled the scene.
Officers investigating the assault are keen to hear from anyone who was in the park or the surrounding area at this time.
The man has been described as Asian, in his late 40s, of skinny build and with black hair. He is believed to have been around 5ft 6ins tall, with a gold tooth in the top left of his jaw.
He was wearing dark blue jeans and a white t-shirt.
Investigating officer DS Jo Martindale said: “This has undoubtedly been a very upsetting and distressing incident for this girl.
“Officers have been out making enquiries today in the area and additional patrols are in place.
“If you have any concerns, then please do not hesitate to approach an officer.
“We want to hear from anyone with information which will assist our investigation.
“No matter how small a piece of information may seem to you, it could be vital to our enquiries.
“If you were in Mayflower Park yesterday afternoon and saw anything suspicious, or saw a man of this description, then please get in touch.
“Also, if the description seems familiar to you, then we want to hear from you.”
Anyone with information can call 101, quoting the reference 44210271113. Alternatively, you can call Crimestoppers anonymously on 0800 555 111.