The 26-year-old victim injured her jaw in the altercation with three white males wearing hooded tracksuits and baseball caps at Oakwell Hall Country Park in Birstall.
West Yorkshire Police is keen to speak to anyone who was in the vicinity of the park land when the robbery happened, between 4-7.20pm on Friday.
The Green Flag country park extends to 110 acres and includes Oakwell Hall, a 16th century manor house, and is a popular visitor destination.
One of the suspects had a short, scruffy beard.
Anyone with information is asked to contact Detective Constable Topham at Kirklees CID via 101.