The victim, aged 16, has cash and mobile phone stolen while trying to help a girl who was attacked.
The teenager was left critically ill after he was battered with a hammer and baseball bats by a masked gang in a Birmingham park.
They stole cash and a mobile phone from him between 4.15pm and 4:30pm on Sunday, May 12.
The boy was rushed to hospital and remained in a critical condition on Monday.
The 18-year-old girl was also punched in the face as she tried to intervene.
Sgt Dave Bebb, from force CID, said:
"This was a nasty attack which has left a boy with very serious injuries.
"Officers are actively seeking to identify those involved and I would urge anyone who can assist with our enquiries to come forward."