The 16-year-old girl had been walking through a nature garden, near to William Street and Lock Lane, at around 7pm on Saturday.
It was then that was she approached by a man in his early 30s, who sexually assaulted her.
Sergeant Tony Shand, of Wakefield District Policing, said: "This was a very distressing incident for the victim and we are doing all we can to trace this man.
"This incident may cause concern in the local community and officers from the neighbourhood policing team are conducting extra patrols in the area."
The suspect is described as white, with light stubble and short dark brown or black hair.
He wore a black top with white stripes down the sleeves, similar in design to an Adidas top, and had a beer belly.
Police said the man was described as speaking with a local accent and was carrying a blue, plastic carrier bag.
He also had a small black dog, possibly a Staffordshire bull terrier, which had a ginger or brown patch on its back and a pink collar.
Sgt Shand said: "I would appeal for anyone who recognises a man matching this description or saw a man similar to this in the area on Saturday evening to get in contact with police."
Anyone with information should contact Wakefield Police on 101, quoting crime reference 13170264014