Police are appealing for information about the robbery in Birkenshaw Bottoms Park off Moor Lane in Birkenshaw, West Yorkshire, on Monday 31 October.
The incident happened between 9.15pm and 9.45pm when the 18-year-old victim was sat on a bench with two friends next to the entrance to the park.
The trio were approached by five males who were all wearing Guy Fawkes masks made famous by the Anonymous movement and the film V For Vendetta, West Yorkshire Police say.
The victim decided to leave but as he stood up he was attacked. He fell to the floor and the attack continued. His mobile phone, a black Samsung Galaxy S6 with a white and black phone case, was also stolen.
As a result of the incident the victim suffered grazing and bruising.
Pc Jack Hodges of Huddersfield CID said: "I would appeal to anyone who witnessed the incident or has information about it or who saw five males leaving the scene to get in contact.
"Or if you have been offered a mobile phone for sale matching the description or know of anyone who has recently got one to call on 101 quoting crime reference number 13160543044."