A 17-year-old boy was kicked and punched near a park in Borehamwood.
Police say the teenager was approached by three black males in Meadow Park at 7pm on August 18.
He was approached from behind before being pushed over, kicked and punched.
His attackers then made off with his mobile phone.
Hertfordshire Constabulary is appealing for anyone with information or who saw what happened to come forward.
Detective Constable Vlad Frolov said: “I would appeal to anyone who was in the park at the time and saw what happened or saw anything which could assist our investigation to please make contact.”
Anyone with information is asked to email DC Frolov at vlad.frolov@herts.police.uk or call 101, quoting crime reference 41/66870/22.
Alternatively, contact Crimestoppers anonymously on 0800 555 111 or via crimestoppers-uk.org.
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