The incident happened at Dewsbury Market between 8.30pm and 9.00pm on October 27, when two 16-year-olds were approached by a group of males aged between 18-20.
The victims were then hit, and assaulted after they fell to the floor.
While they were on the ground the group, all wearing dark-coloured clothing, stole gold-coloured Samsung Galaxy S6 edge mobile phone, a Hublot brown leather wrist watch with a round face and cash.
Both victims were taken to hospital after the attack.
Anyone with information is asked to call police on 101.