The 15-year-old was set upon by the group in Buckie Road in Mayfield, Midlothian, at about 9.25pm on Friday August 4.
The boy was repeatedly kicked by his attackers, before one of them struck him with a bottle.
He suffered a serious facial injury in the incident.
He was taken to Edinburgh Royal Infirmary for treatment and was later discharged, although his wound will require further medical attention.
Police Scotland said the main suspect involved was a white youth, aged about 15 or 16 years old, about 5ft 10in and wearing a dark-coloured hooded top.
Detective Constable Lynn Myles of Dalkeith CID said: “This was a brutal and frightening attack, which has left a 15-year-old with significant injuries.
“We are eager to trace the youths responsible for this incident, particularly the male who used a bottle as part of the assault.
“I would ask anyone who saw this incident, or recognises the description of the suspect, to contact us as soon as possible.
“I would especially appeal anyone who may have filmed this incident on a mobile device to get in touch with us and let us see this evidence.”
Anyone with information can contact police on 101.