The 16-year-old victim was walking in Hove Park with friends when they were approached by a group of around seven people.
It happened at around 4.30pm on Wednesday 7 April.
The victim was punched and pulled off his bike, which was then stolen. The bike (pictured) is an orange and black Scott Scale mountain bike.
The suspects are described as males between 16 to 18 years old, wearing dark clothing and hooded jumpers.
Anyone who witnessed the robbery, saw any suspicious behaviour in the area around that time, or who may have seen the stolen bike, is asked to get in touch.
Information can be reported online or by calling 101 quoting serial 872 of 07/04. Alternatively, you can contact the independent charity Crimestoppers anonymously on 0800 555 111.
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